Proposed minor change to get.offsets() function

Eliminates case dependence for dropping "datetime" column.  If you get water temp data from a function like getTempGLMnc(), the first column header is "DateTime".  Then the first column isn't dropped because of case, and I get a dimension mismatch error when using some of the ts.xxx.xxx time series wrapper functions.  I see from the documentation that wtr is intended to be loaded with load.ts(), but I thought this might still be helpful.
Merge pull request #23 from snortheim/patch-2

Proposed minor change to get.offsets() function