Fix JSLint errors
Make grouped_timeseries cope if series are missing.

Currently grouped_timeseries blows up if one or more series are missing in
the Backdrop response. Instead it should simply parse the series that it
can find, and politely ignore any that it can't.

Extend tests appropriately.
Fix JSLint error
Make grouped_timeshift cope if series are missing.

Currently grouped_timeshift blows up if one or more series are missing in
the Backdrop response. Instead it should simply parse the series that it
can find, and politely ignore any that it can't.

Extend tests appropriately.
Fix JSLint errors
Merge pull request #231 from alphagov/bugfix/handle-missing-data-in-grouped-timeseries

Handle missing data in grouped timeseries