remove direct dependency on thin as the rack implementation

the main reason for this is to allow compatibility with jruby as thin is not really compatible out of the box with jruby.

the change basically removes thin from the dashing.gemspec file and moves it to the dashboard's Gemfile where it is wrapped in a `platform :ruby` block and instead of using 'thin start' it uses 'rackup' to start the server.

initial problems:
 - current projects may revert back to webbrick on up grading as there Gemfile won't have been changed
 - the dashing stop command assumes 'thin' as there doesn't seem to be an equivalent `rackup`, guess may have to use the rack.pid file and kill the process