Added function to explicitly close imap resource

[dharyk]

Calling imap_errors() to clear the error stack and prevent errors and notices from popping up in the output. Any errors are returned as the function result. Function is also implicitly called on class destruction.

P.S.: Related to #94, #69, #62 and #59.

[coveralls]

Coverage decreased (-2.81%) to 87.22% when pulling 38a62d80e6f46713f41e835f59cb0da12e8b0913 on dharyk:explicit_close_stream into ba247b861dbee8151743797e740d99d931d678e4 on tedious:master.

[tedivm]

This isn't passing tests :-/