Notifying users when the sound could not be loaded

A function can be passed as parameter to the property onloaderror,
in the Howl constructor.
Sometimes the user need to know if there was some error when trying to load
the sound, so he can take an action.

Signed-off-by: Thiago de Barros Lacerda <thiago.lacerda@openbossa.org>
Allow passing the file format as a property.
Update AMD define to export Howler and Howl as single module.
Fix issue with chaining on play not working
Merge branch 'master' of https://github.com/thiagolacerda/howler.js into thiagolacerda-master
Update readme with new onloaderror event
Merge branch 'update-amd' of https://github.com/nanek/howler.js into nanek-update-amd
Merge branch 'file-format' of https://github.com/nanek/howler.js into nanek-file-format
Update readme with new format property
Update changelog, minified version and bump to v1.1.1