Minor fixes to allow native gems to load.

- websocket_mask.rb is in lib which in turn then loads websocket_mask.so
  which should reside in the extensions directory external to the gem
  directory at least as of rubygems 2.2.2.

- Changed extensions directory to match gemname as suggested in rubygems
  guide

Simplifies loading the native extension but only tested on ruby 2.1.1
and only in my environment.
Update rake file to build extension in new dir

- Changed name of ext directory to match gemname, updated rake file
  accordingly.

  It would probably make sense just to name the actual extension after
  the gem name as well, Rake would then just do "the right thing"

  Confirmed tests pass on OSX and Linux for C native ext not tested
  Java.
Fix java native extension loading.
Merge pull request #17 from stevenwilliamson/native_ext_loading_fix

Native ext loading fix