Modified events and t-types in DList and Tiles

Added parametric polymorphism :
The events and DLists/Tiles are parametrized by a type 'a which holds
additional informations about the music notes (can be instanciated
with e.g. velovity, pitch...)
Added first stage of implementation  for module DList
Returned to non-polymorphic types for events. Added some implementations in DList.
Added Music files to Makefile
Added simple testing functions, list to DList transforms
Added some implementations.
Added interface/implementation for module Time.

Defines a time-unit that is used for the output of music.
Added interface/implementation for module Music.

Defines the type used to describe musical events.
For now limited to simple notes with pitch and velocity.
Should implement useful descriptors for the rendering of music.
Added implementation for module Tiles
Added normalization functions in module DList
Added function getDur to the interface for Tiles

@return the duration of the input tile
Merge branch 'music_dev' of https://github.com/VLanvin/GL_MPRI_2014 into music_dev