Auto merge of #611 - vityafx:add-pthread-priority-functions, r=alexcrichton

Add schedule params & priority pthread functions

Fixes #609. Provides bindings to `pthread_getschedparam`, `pthread_setschedparam` and `pthread_setschedprio` functions.

P.S. My first PR in such a project.
P.P.S. `libc-test` crashed at building stage with a lot of errors, so I hope `travis` will do a check.
P.P.P.S. These functions are tested to work on Linux (Ubuntu 14.04) with my own crate. I will release it next week, it provides ability to control thread schedule policies and priorities.