Moved the resolveHandler method to a seperate class HandlerResolver. 

The Dispatch class now depends on the HandlerResolverInterface. This allows to people to implement their own resolvers. Basically this allows to use Phroute more easily with a IoC Container.

Notes for the author @joegreen0991 

Note: I do not know what this does to the performance. Apart that 2 new files need to be included I cannot see why this would hurt. The second option would be to make resolveHandler a protected method. This allows the method to be overwritten. My feeling is however that resolving the handler is a seperate responsibility and therefore should live in its own class.