Add support for error prone as native compiler option

Created by: davido

Fixes #994 (closed).

Summary: http://errorprone.info is a static analysis tool for Java that catches common programming mistakes at compile-time. With this change it can be permanenty or instantly activated as default compiler. Moreover, java_library rule is extended as well.

The configuration option: java.error_prone_javac is considered to be experimental and is not documented.

Test Plan:

1. Permanently activate error prone: add this line to .buckconfig:

   [java]
     error_prone_javac = true

2. Instantly activate error prone:

  $ buck build --config java.error_prone_javac=true <rule>

3. Activate error prone per rule base:

java_library(
    name = 'foo',
    error_prone_javac = True,
    [...]