Javadoc for GDX Files

Add a note about how different some files are (compressed inside an
.apk for example).  Try out a "package-info.java" file for documenting
package-level ideas.
Replace @throw with recognized @throws throughout FileHandle.java
Merge pull request #232 from bitiotic/docfixes.files

Javadoc fixes for com.badlogic.gdx.files