Replaced HTML::Scrubber with HTML::Restrict

HTML::Scrubber was leaking like crazy, causing Lixuz to just grow and grow.
HTML::Restrict duplicates its functionality very closely, and does not leak.
Added a streaming mode for lixuz-sendmail.pl / L::HM::Mailer

In streaming mode data is sent to lixuz-sendmail.pl while it is being
generated. The result is that Lixuz needs to hold less data in memory,
ensuring that it doesn't grow in proportion to the number of mails
that it needs to send.
Added autoPrepareNewsletter

Prepare returns data instead of sending it, which lets the caller
handle the actual sending, ie. through L::HM::Mailer's streaming
mode.
Added a parameter to force all subscriptions to be processed
Use streaming mode when sending newsletters