Support for Java Serialization on Rex

The format specs here: https://docs.oracle.com/javase/8/docs/platform/serialization/spec/protocol.html

The support here isn't complete, but is good enough for my purposes atm :) Probably I'll modify it in the next days/weeks. But I think it's good enough for PR. So I can get feedback if there are stoppers.

Verification
--------------

- [x] Be sure travis is green
- [x] Run specs locally and ensure they pass

```
$ rspec spec/lib/rex/java/serialization/model/*
Rex::Java::Serialization::Model::Annotation ............
Rex::Java::Serialization::Model::Field ..............
Rex::Java::Serialization::Model::NewEnum ......
Rex::Java::Serialization::Model::NewClassDesc ...................
Rex::Java::Serialization::Model::ClassDesc .....
Rex::Java::Serialization::Model::Stream .................
Rex::Java::Serialization::Model::LongUtf .............
Rex::Java::Serialization::Model::Utf .............
Rex::Java::Serialization::Model::NewObject .......
Rex::Java::Serialization::Model::NewArray .........................................
Rex::Java::Serialization::Model::BlockDataLong .............
Rex::Java::Serialization::Model::BlockData .............

Finished in 0.20879 seconds
173 examples, 0 failures

Randomized with seed 22548

Coverage report generated for RSpec to /metasploit-framework/coverage. 18080 / 95914 LOC (18.85%) covered.
```