Added option to disable certificate validation

If a IMAP-server is using a self-signed certificate, this patch adds a "verify_cert" option to the module that is by default set to true (old behavior), but if set to false it will disable the verification check. The result is that IMAP TLS with self-signed certificate will not crash logstash.
Added option to disable certificate validation
Added warning when registering insecure state
Merge pull request #1092 from subsection/master

Added option to disable certificate validation in the IMAP input