[WIP] Add library support to the plugin

This does the following:
- ensures that only files terminating with ```.go``` will be used in the parsing process
- allows files from ```OrderRootType.SOURCES``` to be parsed, not from ```OrderRootType.CLASSES```
- remove the unused function from the file, thus a small cleanup
- adds library support to the plugin

~~According to the [IntelliJ sources](https://github.com/JetBrains/intellij-community/blob/master/platform/projectModel-api/src/com/intellij/openapi/roots/OrderRootType.java#L57), ```OrderRootType.SOURCES``` should be the place where the source files are found, thus the ```.go``` files, while ```OrderRootType.CLASSES``` is the place where the classes are found, thus the ```.a``` files.
As we currently don't parse ```.a``` files, we should change this to allow a more native behavior. Also it has the nice side effect that we can support again creating an IntelliJ project inside the GOPATH and still use the GOPATH as it is.
I'm also pending clarification on this one on Gitter, to be sure I'm not wrong.~~

This adds library support to the plugin and thus fixes everything.