Adds testing scripts and Gson as first library.

[advayDev1]

Fixes #21
Fixes #19

Do #22 first.

[brunobowden]

I think the directory names should follow the package name convention, replacing : with -. In this case:

com.google.code.gson-gson

[advayDev1]

Done

[brunobowden]

Please respond to the comments. Close to LGTM.

[brunobowden]

Note that the prior LGTM was a comment on a related PR, not this specific one. Ping me again when thsi is ready for another round.

[advayDev1]

@brunobowden ptal - I will flatten the 2 commits once LGTY

[brunobowden]

This shouldn't be done by default... if it is really needed, I was expecting a comment like:

// Suppress existing filename collision:
// src/main/abc/File1.java
// src/main/xyz/File1.java

Are you using no-package-directories for the translation? If there is a real collision going on, then I think that prefixes.properties can help fix that.

[advayDev1]

1. prefixes.properties does not affect the filename, just the class name. Also, these libraries in our repo should never use prefixes.properties. Unless all transitive forward and backward dependencies of a library use the same prefixes.properties files, you are not guaranteed they will be able work with each other.
2. We aren't using no-package-directories, however the plugin checks for filename collisions regardless of no-package-directories. That is a bug in the plugin, filed: filenameCollisionCheck should default to the value of no-package-directories j2objc-gradle#467 - so until that is fixed it should indeed be set to false - because files in distinct directory do not indeed collide.

[advayDev1]

btw, package-info.java exists in multiple packages in all of these libraries.

[brunobowden]

LGTM after addressing last 3 comments.

[advayDev1]

all done - merging.