Song class refactoration
New classes refactoration
Insert part and period numbers in MusicUnit object
Small fix
Song and MusicUnit classes connections
Functions names improvements
Fixes Song without music21 objects creation
Uses a better word for pickle save argument
Better filename handling
Update for data package and new Song/MusicUnit classes
Move webpage auxiliary functions from utils to webpage package

* utils package doesn't need modules/packages that includes music21.
  It's faster to call utils module now.