Online membership signup - contribution amounts field should not be required + various notices

The main issue is that when 'Separate Payments' flag is FALSE we are requiring the user to enter a contribution amounts value. Contribution amount should never be required UNLESS 'Require Membership Signup' is FALSE AND the user has chosen 'No thank-you' for the membership field.

There are also notices being thrown on certain combinations of flags, and the label for the contribution amount field(s) is not correct.

This wiki page provides details on the case combinations and resulting bugs:
http://wiki.civicrm.org/confluence/display/CRM/Online+membership+signup+-+case+grid+for+testing