ENH: Allow timestamp and data label to be set when exporting to Stata
closes #6545

Added code which allows the time stamp and the data label to be set using
either StataWriter or to_stata.  Also simplified reading these values using
StataReader by removing null bytes from the string values read.

Added basic test for both.

Also fixed one small bug where variables could be stored using Stata reserved
words.

Addresses the issues in #6545.  Also one very small fix for variables that have name conflicts with reserved words.
 @bashtage gr8! FYI keep in mind if you need/want to break back compat should be brought up (not that this change does, and your previous change was essentially a bug-fix)
 @jreback I am conscious that breaking backward compat is generally a bad thing that should be avoided if possible.

On the upside, I finally figured out how to rebase correctly, so that I only get 1 commit when I want only 1 commit.  

BTW, is there something i did to not get a TravisCI build?   It failed the first time on the 3.x versions due to a silly leading-0 issue, but I wanted to check that it all passes.
 https://travis-ci.org/pydata/pandas

I _think_ that you have to enable travis to access your github account to directly look on travis (e.g. then you can look up all of your builds). This is a 'default' build in that it automatically builds all PR's.
 I don't think this fail is me... looks like random network timeout.  Does it need to run until it passes?  And if do, is there a way to do this would a fake commit.
 nope...that is a random failure....minor typo other wise looks good
 can you rebase on current master and force push....had some issue with the caching of the deps...should be resolved now
 Rebased
 thanks!
