Backported from python 3 to python 2.7.

Significant adjustments to date handling, and some other minor tweaks to work
with 2.7.  Also tidied up to conform to PEP8.
Merged python2.7 branch to make python3 the option, and python 2.7 the master, and removed unecessary import
Changes to produce multiple plots from one set of data

Added caching to get_data
Abstracted out independent variable name from make_plot, allowing code re-use,
    including error handling in case independent variable isn't in returned
    data.
Included a deliberately non-existent independent variable to verify that error
    handling above works as expected.
Added comments to hard-coded units to remind that they're also used in plot
    labels so need updating there too.
Stopped showing the plot in GUI
Added second plot - windspeed - to main function.