protobuf support for jruby

JRuby support via native extension.

It's basically a wrapper around ```DynamicMessage```

The tests are changed to use minitest instead of test/unit.

There are a few differences/bugs than the c implementation:

* Enum value cannot take arbitary integer
* Non-repeated fields are copied, and cannot be modified, this goes directly into `DynamicMessage.Builder`
* Serialize circle is allowed
* Corecursive messages cannot be inspected or compared, seems ```DynamicMessage``` cannot tell the difference between an empty message and the default instance.