Asynchronous emails: added a retry mechanism useful in case your SMTP server restricts the number of emails that can be sent over a period of time (usage: broadcasting a newsletter). The mechanism is not specific to sending email as it is implemented at the AsyncTask level.

git-svn-id: http://svn.code.sf.net/p/itop/code/trunk@3047 a333f486-631f-4898-b8df-5754b55c2be0