Tidy up titles and descriptions

Here are a few small changes to text and titles to tidy up:

1. The title of the menu item should be simply 'Scheduler' instead of 'Scheduler module settings'. Most other modules just have their name so for standards it is best to match them, and the words 'module settings' are not required.
2. Change the description to actually say what can be done, because 'allow site admins to configure scheduler' gives no real help.
3. Now that we have tabs, we do not need to set the titles of the pages. Other modules do not have different titles for the tabs so remove the call to drupal_set_title()
4. Scheduler's own cron is referred to by several names, 'internal', 'light weight' and 'lightweight'. Most commonly it is the single word 'Lightweight' so for consistency, I suggest we change to that

patch to follow