Add notification's emails HTML alternative + subject translation

[DylannCordel]

Related to #167

• Emails are now sent in text + an HTML alternative (two templates available). Test added.
• Email's subject was outside the "for loop" with user's locale activation and emails was sent with last post author locale. Correction and tests added.

[zopieux]

Did you mean send_html_mail here? AFAICT the mass one will always trigger the exception as it's not imported/defined anywhere before.

[zopieux]

Don't forget to wrap text within {% autoescape off %} in all the plain-text mail templates. If you don't, the user Belieber<3 will be rendered as Belieber<3 in the mail client.

[DylannCordel]

Thanks for your feedback @zopieux
For the try/except, I removed it : if user set pybb to use mailer but that this one can not be imported, IMHO it's better to not intercept the exception.

[DylannCordel]

@GeyseR : could you restart the travis buid please ? It failed becauseof a network bug from travis and/or github. Thx !

[DylannCordel]

rebased with master

[DylannCordel]

up...