Add ability to set custom log functions to mail-notifier and imap module.

[mpelekh]

• add ability to set custom debug for mail-notifier and imap module
• add dbg message in stop() about imap.state
• add ability to set mail-notifier's instance debug function
• fix wrong dbg message when imap connection closed
• update Readme.md

[jcreigno]

Thanks for your contribution.

[mpelekh]

Thanks for mail-notifier.