Add support for JRuby-1.7.{5,6}

The method RubyString.encode_bang(ThreadContext) was removed, so call
the array version with an empty array instead.
Rewrite Unpacker with the new decoding implementation
Remove .rvmrc, you should have your own local .ruby-* files instead
Remove the old implementations and dependencies
Remove the tests for the old extensions (symbolizations and encodings)

We'll have to see if they should be in v2.0 at all