CB-6698: Support library references for Android via the framework tag

The framework tag can be contain the following attributes:
	
* **src** - (**required**) relative path to the directory containing the referenced library project.
* **custom** - (optional) specifies how to treat **src**. If **true** it is a relative path from the application project's directory, otherwise -- the Android SDK directory.
* **parent** - (optional) relative path to the directory containing the project to which to add the reference. The default is the application project.

Example: A plugin that installs a library (**FeedbackLib**) which refers another library from the Android SDK (**appcompat**)

```xml
<source-file src="src/android/FeedbackLib" target-dir="./" />

<framework src="FeedbackLib" custom="true" />
<framework src="extras/android/support/v7/appcompat" parent="FeedbackLib" />
```