Fix issues with the router

Fix #1938, #1961; reverts #1078.

Bonuses:
  * Relaxed the requirement for the route to start with a slash (see https://github.com/phalcon/cphalcon/blob/1.3.0/ext/kernel/framework/router.c#L175 and @xboston 's [test](https://github.com/phalcon/cphalcon/issues/1938#issuecomment-33894753))
  * Merge slashes when prefix ends and pattern starts with a slash (`$this->setPrefix('/test-class/')->add('/test/{id:\d}')` will result in `/test-class/test/1`, not `/test-class//test/1`)