Runner works even when engine is mounted

The correct path to the specs needs to be specified. That path changes when Konacha is mounted. 

The Rack environment variable SCRIPT_NAME contains the mount path if Konacha is mounted. Otherwise it is empty. This variable is also available when running under webrick.

This pull request adds a path helper that uses this variable to generate the correct path to the specs.

After these changes are applied, Konacha can be run as a mounted engine (from within the Rails app itself), or from either of the rake tasks: `rake konacha:run` or `rake konacha:serve`.