Fix getTime and getFreq
Add mention of MeasurementSet interface to README
Re-write c++ getColumn and putColumn to use templates

This reduces code duplication
Implement putKeyword for strings
Add tests for getFreq (exporting putKeyword!)
Refactor getKeyword and putKeyword! to be more generic

This will make it easier to get and put keywords that are not strings,
which will be implemented as soon as I need it (ie. not yet)
Merge pull request #11 from mweastwood/improvements

Various fixes and improvements