return pandas dataframe from selectData instead of recarray (#8)
replace time/distance conversions to take pandas dataframes

Remove old duplicate versions of functions converting unixms to
features. New code assumes pandas Series objects as inputs, so may
want to check for scalars. Still had to use numpy datetime casting in
getWeekSecs since pandas only handles datetime64[ns] types and casting
is a lot faster than operations finding start of week on a vector.

Also removed offset between value returned by weekday and
datetime64[W] casting. The latter week starts on Thursday (since Jan 1
1970 was a Thursday) so I shifted everthing so Monday = 0.
add pandas and remove geopy from dependencies