Add Bundler. Tested on Ruby 2.

Updated Rakefile Hoe block so that
running "rake" successfully runs a test
task for the existing test suite.

Hoe and newgem could likely be removed 
completely in favor of Bundler, but I thought
I'd propose this smaller step first.
Merge pull request #1 from andyatkinson/bundler_plus_fix_Rakefile

Add Bundler. Tested on Ruby 2.