num_cpus library evaluation tracking issue

[dmfutcher]

Tracking issue for issues coming out of the num_cpus Libz Blitz evaluation.

• Expand crate level documentation
• Add example to get() docs
• Add example to get_physical() docs
• Improve get_physical() documentation
• Improve get() documentation
• Use a Result for get_physical() ?

[derekdreery]

The PR #61 would close these if accepted.