CAtomicBoolean and CAtomicFixnum

This PR revisits earlier experiments with native C extensions in PRs #55, #59, and others. It completely reimplements `CAtomicBoolean` and `CAtomicFixnum` based on the merge of `Atomic` in PR #109. This implementation is much better because it is optimized for multiple operating systems. It also includes much better Yardoc documentation. A few simple benchmarks were run to give an idea of the performance improvements.

       Testing with ruby 2.1.2
       Testing with Concurrent::MutexAtomicBoolean...
         2.790000   0.000000   2.790000 (  2.791454)
       Testing with Concurrent::CAtomicBoolean...
         0.740000   0.000000   0.740000 (  0.740206)

       Testing with jruby 1.9.3
       Testing with Concurrent::MutexAtomicBoolean...
         5.240000   2.520000   7.760000 (  3.683000)
       Testing with Concurrent::JavaAtomicBoolean...
         3.340000   0.010000   3.350000 (  0.855000)

       Testing with ruby 2.1.2
       Testing with Concurrent::MutexAtomicFixnum...
         3.130000   0.000000   3.130000 (  3.136505)
       Testing with Concurrent::CAtomicFixnum...
         0.790000   0.000000   0.790000 (  0.785550)

       Testing with jruby 1.9.3
       Testing with Concurrent::MutexAtomicFixnum...
         5.460000   2.460000   7.920000 (  3.715000)
       Testing with Concurrent::JavaAtomicFixnum...
         4.520000   0.030000   4.550000 (  1.187000)