[exif2timestream] support giving "now" as a date

This allow one to set end date to be "now", and then you don't have to
change the date in the config file until the expt acutally ends.
[setup.py] bump version to 0.2.7
[config_dates] add a "now" end date config to test
Merge pull request #16 from kdmurray91/dev

Enable "now" as a date constant