Add unit-tests for gson.

[advayDev1]

Also adds a common script to get test repos from github.
Issue #30

Do #24 first please.

[advayDev1]

Lots o' j2objc-only test failures here:

  Standard Output:
  JUnit version 4.11
  ..E.E.E.E.E....................E.E................E..........................................................................E.E.........E...E.E....E.E..................E.......E...
  Error Output:
  Sep 28 09:02:07 Traviss-Mac-443.local main[2423] <Notice>: Loaded time zone names for "" in 62ms (62ms in ICU)
  Sep 28 09:02:07 Traviss-Mac-443.local main[2423] <Notice>: Loaded time zone names for "en_US" in 32ms (31ms in ICU)

To be investigated some other time...