fixes issue #252

A patch to extend a cookie based session, when a user accesses sync gateway after 10% of the session expiration has elapsed, the original session ttl is added to the current system time and the session and cookie are updated and new cookie is returned to the client.

A new test has been added to the admin api
I'm concerned about any existing session docs in the bucket at the time that SG is upgraded -- they won't have a `ttl` property, so the `Ttl` string will be empty. Pretty sure that means ParseDuration will return a 0 duration, which looks like it'll have some bad effects down below.
Probably you should have duration default to some reasonable value (one day?) if it's zero or if ParseDuration returned an error.