GH9570 allow timedelta string conversion without leading zero
See https://github.com/pydata/pandas/issues/9570

This commit allows for the `Timedelta` class to take a construction like `Timedelta('0:00:00')` or `Timedelta('00:00:00')`.

Test: https://travis-ci.org/chrisgilmerproj/pandas/builds/58278744
 Could you add a release note in `doc/source/whatsnew/v0.16.1`?
 Yep, will do!
 Test: https://travis-ci.org/chrisgilmerproj/pandas/builds/58287006
 This now supports the following:

``` python
In [1]: from pandas import Timedelta; Timedelta('-01:00:00')
Out[1]: Timedelta('-1 days +23:00:00')

In [2]: from pandas import Timedelta; Timedelta('-1:00:00')
Out[2]: Timedelta('-1 days +23:00:00')

In [3]: from pandas import Timedelta; Timedelta('00:00:00')
Out[3]: Timedelta('0 days 00:00:00')

In [4]: from pandas import Timedelta; Timedelta('0:00:00')
Out[4]: Timedelta('0 days 00:00:00')
```
 All right, tests have passed @TomAugspurger @jreback 
 Looks good to me. I'll let @jreback handle the merges today (unless he says otherwise).
