Added 'enabled' function

[someshinyobject]

This will allow users of the library to quickly check if cookies are enabled on the client without having to maintain an external function to do it for them.

$.cookie.enabled //returns a boolean to check

A boolean value check was also added to the unit tests

[someshinyobject]

Travis CI build failed due to a failure in loading saucelabs

[carhartl]

Thanks for the PR. navigator.cookieEnabled isn't implemented cross-browser though, thus not going to merge.