SPARK-1319: 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.

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.
Avoid Option while generating call site

This is an update on https://github.com/apache/spark/pull/180, which changes the solution from blacklisting "Option.scala" to avoiding the Option code path while generating the call path.

Also includes a unit test to prevent this issue in the future, and some minor refactoring.

Thanks @witgo for reporting this issue and working on the initial solution!

Author: witgo <witgo@qq.com>
Author: Aaron Davidson <aaron@databricks.com>

Closes #222 from aarondav/180 and squashes the following commits:

f74aad1 [Aaron Davidson] Avoid Option while generating call site & add unit tests
d2b4980 [witgo] Modify the position of the filter
1bc22d7 [witgo] Fix Stage.name return "apply at Option.scala:120"