Restore numba functionality for numba >= 0.12.1

try_numba now treats numba 0.12.0 as the exceptional case, and raises a warning on import.
Merge pull request #88 from nkeim/return-of-numba

Restore limited numba functionality for numba >= 0.12.1
BUG/PERF: Re-enabled numba usage by default.
Merge pull request #90 from nkeim/return-of-numba

BUG/PERF: Re-enabled numba usage by default.