Make handy shortcut for accessing example videos and the notebook

[balintlaczko]

This would generally make it easy to access the files that "ship" with musicalgestures. It also happens to be very handy for testing. :-) #205

For example the path for the infamous dance.avi could be used as musicalgestures.examples.dance:

import musicalgestures
dance = musicalgestures.MgObject(musicalgestures.examples.dance, starttime=5, endtime=15)

If we don't have this, the user has to manually probe where musicalgestures is installed, and then (with knowing the file names to look for) construct the path. Can be done, but it's not smooth. musicalgestures.examples.file could just instantly do this for all the exampleswe have (so far the 2 videos and the notebook).