Support pgRouting-2.0 functions.

Supported functions are written in the following site.
https://github.com/sanak/pgRoutingLayer/wiki

Modification summary is as follows:
* Add node/edge pickable interface("+" button). (I referred QGIS-1.8 C++ plugin.)
* Add each functions' selectable interface.
* Move original "Run"(to Layer) function to "Export" button. (Because some functions(tsp, drivingDistance .etc) can't support "to Layer" function.) 
* When clicking "Run" button, then its result will be displayed as temporary rubber band.

I don't have much confidence about script(python) and sql(especially rubber-band generation part),
so any modification(or refactoring) is welcome.

Thanks,