Built-in comparator(s) in RocksJava

Extended Built-in comparators with ReverseBytewiseComparator.

Reverse key handling is under certain conditions essential. E.g. while
using timestamp versioned data.

As native-comparators were not available using JAVA-API. Both built-in comparators
were exposed via JNI to be set upon database creation time.
Changed name of ReverseBytewiseComparator based on review comment
Merge pull request #316 from fyrz/ReverseBytewiseComparator

Built-in comparator(s) in RocksJava