java_binary: optionally suppress manifest merging from dependent JARs

For core/plugin pattern MANIFEST.MF file is used to define meta information
used by core system to load the plugin.  In this scenario is important to
have only manifest entries in the resulting artifact that were provided in
manifest_file paramter to java_binary() function.

This change extends java_binary() with additional parameter:

  manifest_pristine

If this parameter is passed, then no merging take place.

This fixes bug #86.

Test plans: https://github.com/davido/buck_test