Add support to merge service Descriptor files in java_binary rules

Created by: kageiit

Buck currently provides an option to merge manifests when building java_binary rules, but I am not seeing an option to merge service manifest files as described in http://imperceptiblethoughts.com/shadow/#controlling_jar_content_merging

Java libraries often contain service descriptors files in the META-INF/services directory of the JAR. A service descriptor typically contains a line delimited list of classes that are supported for a particular service. At runtime, this file is read and used to configure library or application behavior.

Multiple dependencies may use the same service descriptor file name. In this case, it is generally desired to merge the content of each instance of the file into a single output file.