Builds two gems from one repo.

This update will now build two gems from within one repo.

### Building

Commands to build the gems:

* `rake build:core` under MRI or Rbx will build the pure-Ruby `concurrent-ruby` gem
* `rake build:core` under JRuby will be the Java-specific `concurrent-ruby` gem
* `rake build:ext` under MRI will build the `concurrent-ruby-ext` gem with uncompiled C extensions
* `rake build` under MRI will build the aforementioned `concurrent-ruby` and `concurrent-ruby-ext` gems
* `rake build` is an alias for `rake build:core` under JRuby or Rbx
* The [rake-compiler-dev-box](https://github.com/ruby-concurrency/rake-compiler-dev-box) scripts have not yet been updated

### Installing

The `concurrent-ruby-ext` gem lists `concurrent-ruby` as a runtime dependency. Installing `concurrent-ruby-ext` will cause `concurrent-ruby` to be installed.

Installing `concurrent-ruby` alone will install the JRuby-specific gem under JRuby. It will install the pure-Ruby gem under any other platform.

### Using

All the user ever needs to do is `require 'concurrent'` (or a load a subset of the gem, as in `require 'concurrent/atomic'` or `require 'concurrent/atomics'`) and the C extensions will load *if an only if* running under MRI *and* the `concurrent-ruby-ext` gem has been installed.

### Testing

1. Under MRI run `rake build` to create 2 gems
    * On any platform run `rake native gem` to build the pre-compiled extension gem
1. Under JRuby run `rake build` to create 1 gem
1. Under MRI install 2 (or 3) gems
    * `gem install pkg/concurrent-ruby-0.8.0.pre1.gem`
    * `gem install pkg/concurrent-ruby-ext-0.1.0.pre1.gem`
    * Optionally: `gem install pkg/concurrent-ruby-ext-0.1.0.pre1-x86_64-darwin-14.gem` (as appropriate for your platform)
1. Under JRuby install 1 gem
    * `gem install pkg/concurrent-ruby-0.8.0.pre1-java.gem`
1. Under Rbx install 1 gem
    * `gem install pkg/concurrent-ruby-0.8.0.pre1.gem`
1. Under MRI, JRuby, and Rbx run the following scripts and note the differences
    * `ruby examples/benchmark_atomic_boolean.rb`
    * `ruby examples/examples/benchmark_atomic_fixnum.rb`
    * `ruby -Ilib -Iext examples/bench_atomic.rb`

I would release core and ext always together. In my mind they are one library using two gems to deliver it to users. It will also avoid confusion what should be used together.