Move CPUS_PER_TASK to TaskSchedulerImpl as the value is a constant and use it in both Mesos and CoarseGrained scheduler backends.

Thanks @kayousterhout for the design discussion

Author: Shivaram Venkataraman <shivaram@eecs.berkeley.edu>

Closes #219 from shivaram/multi-cpus and squashes the following commits:

5c7d685 [Shivaram Venkataraman] Don't pass availableCpus to TaskSetManager
260e4d5 [Shivaram Venkataraman] Add a check for non-zero CPUs in TaskSetManager
73fcf6f [Shivaram Venkataraman] Add documentation for spark.task.cpus
647bc45 [Shivaram Venkataraman] Fix scheduler to account for tasks using > 1 CPUs. Move CPUS_PER_TASK to TaskSchedulerImpl as the value is a constant and use it in both Mesos and CoarseGrained scheduler backends.