More sugary Foxx exports

The current API for `require('org/arangodb/foxx/manager').mountedApp('/mount/point')` is a good start, but it could be friendlier. Foxx Apps don't normally mess with the `manager` module and the leading slash four mount paths can easily be missed (I can not think of a situation where it should not be provided).

This change allows using `require('org/arangodb/foxx').app('mount/point')` and cleans up missing/redundant slashes automagically, providing a more natural syntax.