Added support for RubyGems >= 2.0.0

RubyGems changed a bit internally with version 2.0.0 which broke the gem. `Gem::Builder` has been replaced by `Gem::Package`.

I modified `Gem::Compiler` to use `Gem::Package` instead of `Gem::Builder` if `Gem::VERSION` is >= 2.0.0.

`Gem::Installer` now explicitly requires `:build_args` to be passed to it whereas before it would implicitly infer them. I modified `Gem::Compiler` to accept an options **Hash** as a second argument rather than just a **String** with the output dir.

I tested the changes using the `mysql2` gem. I was able to compile/package the binary gem on Ruby 1.9.3p327 with RubyGems 1.8.24 as well as Ruby 2.0.0p0 with RubyGems 2.0.3.

Tests were also modified to reflect the changes in the code. All tests pass.

[![Build Status](https://travis-ci.org/mgoggin/gem-compiler.png?branch=master)](https://travis-ci.org/mgoggin/gem-compiler)