Fix varname in percentage
Adding an event module

Previously event were declared in views/mail.py, now we created an event
package and moved the content of mail in tasks and utils.
Change event import and add an attribute

In order to provide the pdf output of a document while sending the mail
message, we need to pass the html output of the doc to the fired event.
Add an utils module
Send attachment with the mail

We now are able to attach the pdf output of a document when sending an
event.
Add the customer in the mail subject
Move the format_mail in the utils module
Ref #39 : Envoi de mail à la validation/au paiement des notes de frais
Add an __init__.py file to make events a package
Fix #39: event notification on expense status change

This way we can send emails when an expense is
validated/invalidated/resulted.