Poll module fixes - due to recent api changes

This patch includes a few fixes caused by recent changes to the Drupal api:

• Fixed a problem with the 'cancel your vote' form on the node page appearing as just Array due to recent changes with drupal_get_form()
• Fixed a problem with the 'poll' page producing php errors because arguments to theme_pager() changed recently - this api change not documented in the 6.x to 7.x changes list btw.
• Added a 'Your vote has been cancelled.' message when a user cancels their vote (minor usability improvement and used in forthcoming tests).

Cheers,
Stella