Added gwpy.signal.window module to help with FFT routines

This PR adds a new module gwpy.signal.window which provides two simple routines

• canonical_name: translates arbitrary window name into one understood by scipy (e.g. ksr -> kaiser)
• recommended_overlap: uses Heinzel, Rüdiger, & Schilling [2002] to return the recommended overlap for a given FFTlength and window combination

The recommended_overlap method is now used to set the default overlap for all TimeSeries FFT routines, unless specified by the user.