Add action hooks to provide better compatibility with other plugins.

The `do_action()` hooks allow site owners to write custom code that
triggers upon various WP-Polls events. This is useful when, for example,
a site owner is running a caching plugin that needs to be told to clear
the cache for a given page when a poll on that page is updated.

See also: http://codex.wordpress.org/Function_Reference/do_action

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See also, related WordPress.org Support Thread regarding Quick Cache and WP-Polls: http://wordpress.org/support/topic/how-to-get-it-work-with-wp-polls-plugin