Added gradle-based builds

Removed Robolectric/Junit 4 testing stuff.  Doesn't work well with gradle right
now.  While I might be annoyed at that, Maven unit testing of AARs is in an
even worse state right now.
Added gradle wrapper
Imported TestDateTimeZone from joda-time

For properly testing the library!
Removed some tests which fail due to differences in SDK
Gradle check now works

Skips an error that was aborting the build, since the error is not a
showstopper.
Removed Maven pom files

Since I'm switching to gradle I don't need these anymore
Added gradle-mvn-push support, for pushing aar
Updated README

- Moved Usage to bottom

- Added Maven Central dependency info
Add task and instructions for including updated timezone data