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
handle kDelete type in cuckoo builder

Summary:
when I changed std::vector<std::string, std::string> to std::string to
store key/value pairs in builder, I missed the handling for kDeletion
type. As a result, value_size_ can be wrong if the first add key is for
deletion.
The is captured by ./cuckoo_table_db_test

Test Plan:
./cuckoo_table_db_test
./cuckoo_table_reader_test
./cuckoo_table_builder_test

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24045
Merge pull request #316 from fyrz/ReverseBytewiseComparator

Built-in comparator(s) in RocksJava
Add AUTHORS file. Fix #203
use GetContext to replace callback function pointer

Summary:
Intead of passing callback function pointer and its arg on Table::Get()
interface, passing GetContext. This makes the interface cleaner and
possible better perf. Also adding a fast pass for SaveValue()

Test Plan: make all check

Reviewers: igor, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24057