support more than one email per customer (coma is a delimiter)

usefull when customer want to receive messages (eg. with invoices)
to more than one e-mail account
add %period_begin(FORMAT) and %period_end(FORMAT) in invoices and liabilities description
to be sure that attached invoices are generated in your language
Merge branch 'master' of https://github.com/lmsgit/lms
Update changelog
Merge pull request #296 from kcem/master

minor improvements and fix