Added retry logic for common connection errors

This addresses bug #74 and some typical issues I've seen connecting to IMAP servers.

Retry logic has been added to the IMAP receiver for Net::IMAP::ByeResponseError and Net::IMAP::NoResponseError exceptions.

Retry logic has been added to the polling loop for generic connection errors, including EOFError. The logic for both sets of retries is the same.

Tests have been added/modified for this functionality.