Wrap test framework dependency, change to Minitest

* Change to use Minitest instead of Test::Unit. It replaced Test::Unit
  in the stdlib in v1.9, so might as well use it explicitly.
* Added a Fastly::TestCase class that wraps Minitest and updated
  the existing classes to use this wrapper. This enables the framework
  to be changed easily at a later date if required.
* Removed the inline calls to require RubyGems and Test::Unit.
* Removed the file munging for requiring {common,helper}.rb - using
  Rake::TestTask sets the load path correctly and also allows
  individual tests to be run.
* Added a test against the Minitest version to accommodate >= 5 for the
  Fastly::TestCase class.
Map method name inconsistency with an alias
Merge pull request #23 from sgerrand/wrap-testing-dependencies

Wrap test framework dependencies and switch to minitest