Remove unused imports from period.pyx
Replace isinstance checks with more efficient versions from util;

Remove redundant checks for isinstance(..., (timedelta, Timedelta)) since
Timedelta subclasses timedelta

The main goal is to reduce the inter-dependencies between lib/tslib/period(/frequencies/offsets).
an asv run for period benchmarks is useful for proving claims here. a simple posted timeit on performance claim is also useful to the causal reader. I've been [instructed](https://github.com/pandas-dev/pandas/issues/17234#issuecomment-325048715) to use the `util` versions wherever possible.

No idea what the Travis error means, so am inclined to drop it.  This for now.  asv will be easier if/when #17331 is merged.