Add DxCompile task to android backend

This task gathers the class files of a target and compiles them into a classes.dex.
Dex file is a Dalvik exeecutable file. Once the classes have been dexed, then the app
is ready to be packaged into an .apk.

The classes.dex that is created is output to
pants.d/dex/dex/target.id/classes.dex. The target.id is
simply the package name. This is done b/c every Android target that
is dexed will output a file named "classes.dex". Putting them in
dirs that are named after the target.id means that they will
not clobber each other unless the package names correspond
(which would cause other problems as well).

CLI options allow configuring of VM options and build-tools versions.

This is just the task itself, I will write the test class tomorrow. I wanted to give people
a chance to comment while I worked on them, in case large changes are needed.
If you prefer to wait for the test class, that should be up tomorrow.

Testing Done:
CI.sh passes at 67089e7
Full integration tests pass on local machine.

Travis now passes [0].

[0] https://travis-ci.org/pantsbuild/pants/jobs/31952336

Reviewed at https://rbcommons.com/s/twitter/r/840/