ENH: Let hold_back and last_obs work with dates

Provide functionality so that these can accept either int or dates
Add testing for first_obs and last_obs
Improve appearance of plot
Merge pull request #4 from bashtage/first-last-dates

ENH: Let hold_back and last_obs work with dates
DOC: Fix doc strings to reflect datetime changes

Updated docstrings and examples to show the use of datetimes in
last_obs and hold_back