Change TimeSeries +/- to use seconds, add GTest

@macumber The addition and subtraction of TimeSeries wasn't working that well - I was getting a lot of this sort of thing when adding together series:

2009-Jan-17 07:59:57 0.030163
2009-Jan-17 07:59:58 0.030163
2009-Jan-17 07:59:59 0.030163
2009-Jan-17 08:00:00 0.030163

I changed the addition and subtraction functions to use the dateTimes method and that seems to have straightened things out.