Allow overriding 'Content-Type' header.

This 'Request' objects passed to a client will now have their header list pre-populated with 'Content-Type' and 'Content-Length' headers set. Previously the 'Client' implementations were responsibile for setting this data.
Merge pull request #441 from square/jw/override-content-type

Allow overriding 'Content-Type' header.
Remove unused jarjar plugin declaration.
Merge pull request #443 from square/jw/what-were-you-thinking-george-lucas

Remove unused jarjar plugin declaration.
Prepare version 1.5.0.
Use latest release plugin.
Changes to make JDK 8 doclint happy.
[maven-release-plugin] prepare release parent-1.5.0
[maven-release-plugin] prepare for next development iteration
MockRestAdapter should use ErrorHandler.
Merge pull request #447 from square/jw/mock-error-handler

MockRestAdapter should use ErrorHandler.
Remove double plural in docs.
Add @PartMap annotation.
Merge pull request #470 from ypresto/add_part_map

Add @PartMap annotation.
Lazily construct RxSupport stuff
Merge pull request #474 from square/edenman/lazy-build-rx-support

Lazily construct RxSupport
Add a RetrofitScheduler which is loosely based on the NewThreadScheduler but alows us to provide the Executor.
Merge branch 'chrisjenx-fix/rxjava-0-18'
Prepare for 1.5.1.
[maven-release-plugin] prepare release parent-1.5.1