use current position when starting (forward_command) position_controllers

effort and velocity (forward_command) controllers still use 0.0

The template class now only contains the declaration for ```starting()```. The implementation moved to the respective controllers.

This is related to #104
Can this actually fail ,given that `init` should return false if the requested joints don't exist?.

I say this because `starting()` is a real-time safe method, and exception handling is not a real-time safe mechanism, in the sense that stack unwinding takes a non-deterministic amount of time.