ENH: Added temporal decimation to Morlet analysis

To help minimize memory usage when processing large datasets, added decim
parameter to `single_trial_power` and `cwt`.

This allows me to analyze some datasets where the final result of
`single_trial_power` would not have fit in memory otherwise.  Having `cwt`
perform the decimation before returning the result also sidesteps the issue of
multiple jobs (using joblib) returning large arrays to the parent process that
crashed my Python interpreter.

Includes:
* docstring fix for `single_trial_power`
* bugfix where `single_trial_power` was not calling `cwt` with correct
  arguments when njobs was 1.
* Updated some code to use the decim argument to `cwt` rather than decimating
  after the function returns the data.
* Updated two examples to showcase the decim argument.
Merge branch 'tfr-decim' of https://github.com/bburan-galenea/mne-python into bburan/tfr-decim