Full refactoring

Hello JT Zemp

First: Congrats and keep your good work! base62 is a very useful gem

I made few changes in the base62 gem, waiting your approval

1- I added the Rakefile, now you can run these tasks
1.1 - "rake test" - to run all available tests
1.2 - "rake build" - to build the gem
1.3 - "rake install" - to install the gem
1.4 - "rake release" - to generate a tag on github with the actual version number and publish the gem on rubygems 

2 - I changed the test_base62 to execute a effectively test without use the previous loop