Implement search timeout parameter

This is a proposal to implement timeouts for search requests ([RFC 4511 4.5.1.5](http://tools.ietf.org/html/rfc4511#section-4.5.1.5)). I decided to name the parameter `time` to be consistent with the existing `size` parameter. I still need to figure out a good approach for testing this, but wanted to open this PR for comments.

Fixes https://github.com/ruby-ldap/ruby-net-ldap/issues/10. Note that this is the protocol timeout, and not a hard socket timeout. That can come in a separate PR.