Provide configuration setting to disable email notifications to Activity Assignees

We're getting complaints from folks who don't want the assignee notification behavior:
http://forum.civicrm.org/index.php/topic,10912.msg50466.html#msg50466

For 3.1, let's add a configuration file setting that folks can use to turn this off:

// controls whether email is automatically sent to activity assignees
define( 'CIVICRM_ACTIVITY_ASSIGNEE_MAIL' , 0 );

Default for this config var is TRUE (if constant is not defined in settings file).

Add the config var with the default value = 1 to civicrm.settings.php.tpl so it's present fro new installs.

If the variable value is FALSE, then no email is sent to assignees (for both regular and case activiites).

Also need to check this value in Activity.tpl and only show the 'help' text below IF the value is TRUE:

"A copy of this activity will be emailed to each Assignee."