NSString VFL category

@DAlOG @AndreyMoskvin #19 Check NSString category out and accept. 
There are commits available that resolve #27 and #20. Ignore them. :cat2: 
Also I've add here some convenient ```PVGroup``` addition:
From now on you can put inside ```PVGroup``` argument array simple ```NSString``` for VFL without need of some additions.
mb `pv_vfl` ?