BUG: TimedeltaIndex raising ValueError when boolean indexing (#14946)
 - [x] closes #14946
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Implementing patch suggested by @jreback [here](https://github.com/pandas-dev/pandas/issues/14946#issuecomment-268666453) that prevents raising a `ValueError` when boolean indexing with a `TimedeltaIndex`
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15221?src=pr) is 86.31% (diff: 100%)
> Merging [#15221](https://codecov.io/gh/pandas-dev/pandas/pull/15221?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15221   diff @@
==========================================
  Files           139        139          
  Lines         51091      51093     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          44099      44101     +2   
  Misses         6992       6992          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [0c8442c...b8ac04e](https://codecov.io/gh/pandas-dev/pandas/compare/0c8442c3f5c76c8e5b36ca313d3c38527e9f4b0c...b8ac04ebc84086b6dbe59f02384a9fd00a2b5ecf?src=pr) Moved the tests to `pandas/tests/indexing/test_indexing.py`, created class for TimedeltaIndex, and all green. I didn't find any tests here that tested TimedeltaIndex exclusively, so I didn't group any more under this class.  thanks @mroeschke keep em coming!