More parameters for IMAP

- mail_max_userip_connections: set to default of 10
- imap_idle_notify_interval: set to default of '2 mins'

Using dovecotcfmulti since there are several parameters

Bugfix: mail_plugins should be set inside the 'protocol imap' block
Setting default IDLE notify interval to 29 mins

This is per [RFC 2977](http://tools.ietf.org/html/rfc2177)
Setting default max user IP connections to 3

This is per @mjhas's comments on PR #9

I could not locate any certain default setting for this, besides the "default" of 10 in `/usr/share/dovecot/conf.d/20-imap.conf`; however, it appears to be more of an example than a default value.
Merge pull request #9 from andschwa/userip

More parameters for IMAP