Add LevelDb's Java interface

Summary:
See the wiki below
https://our.intern.facebook.com/intern/wiki/index.php/Database/leveldb/Java

Test Plan: compile test

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D5919

java/leveldb/.gitignore

@@ -0,0 +1,22 @@
+target/
+/var
+pom.xml.versionsBackup
+test-output/
+/atlassian-ide-plugin.x
+.idea
+.*.swp
+.*.swo
+leveldb-c
+*~
+*.swp
+.idea
+.idea/*
+*.iml
+*.ipr
+*.iws
+.DS_Store
+.scala_dependencies
+.project
+.classpath
+.settings
+eclipse-classes