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

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.
Merge pull request #6553 from bashtage/time_stamp-data_label-reserved_words

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