Split of tests requiring live internet connection (#28)

* Split of tests requiring live internet connection

Split of tests requiring live internet connection

You may not always have a live internet connection available during
testing. By moving all tests that require a live connection into a
specific group, they can easily be toggled with `--exclude-group`.

This moves some Stream tests into a group called internet. This group
name reflects the one used by Guzzle:
https://guzzle3.readthedocs.io/testing/unit-testing.html#group-internet-annotation

* Add example of running tests without internet

Author: Zegnat

README.md

@@ -35,6 +35,11 @@ Please see the [official documentation](http://docs.php-http.org/en/latest).
 $ composer test
 ```
 
+It is also possible to exclude tests that require a live internet connection:
+
+``` bash
+$ composer test -- --exclude-group internet
+```
 
 ## Contributing
 

src/StreamIntegrationTest.php

@@ -137,6 +137,16 @@ public function testIsSeekable()
         fwrite($resource, 'abcdef');
         $stream = $this->createStream($resource);
         $this->assertTrue($stream->isSeekable());
+    }
+
+    /**
+     * @group internet
+     */
+    public function testIsNotSeekable()
+    {
+        if (isset($this->skippedTests[__FUNCTION__])) {
+            $this->markTestSkipped($this->skippedTests[__FUNCTION__]);
+        }
 
         $url = 'https://raw.githubusercontent.com/php-http/multipart-stream-builder/master/tests/Resources/httplug.png';
         $resource = fopen($url, 'r');
@@ -154,6 +164,16 @@ public function testIsWritable()
         fwrite($resource, 'abcdef');
         $stream = $this->createStream($resource);
         $this->assertTrue($stream->isWritable());
+    }
+
+    /**
+     * @group internet
+     */
+    public function testIsNotWritable()
+    {
+        if (isset($this->skippedTests[__FUNCTION__])) {
+            $this->markTestSkipped($this->skippedTests[__FUNCTION__]);
+        }
 
         $url = 'https://raw.githubusercontent.com/php-http/multipart-stream-builder/master/tests/Resources/httplug.png';
         $resource = fopen($url, 'r');
@@ -171,6 +191,16 @@ public function testIsReadable()
         fwrite($resource, 'abcdef');
         $stream = $this->createStream($resource);
         $this->assertTrue($stream->isReadable());
+    }
+
+    /**
+     * @group internet
+     */
+    public function testIsNotReadable()
+    {
+        if (isset($this->skippedTests[__FUNCTION__])) {
+            $this->markTestSkipped($this->skippedTests[__FUNCTION__]);
+        }
 
         $url = 'https://raw.githubusercontent.com/php-http/multipart-stream-builder/master/tests/Resources/httplug.png';
         $resource = fopen($url, 'r');
@@ -207,6 +237,7 @@ public function testRewind()
     }
 
     /**
+     * @group internet
      * @expectedException \RuntimeException
      */
     public function testRewindNotSeekable()