adding visible prop for lastIdentity and short-circuit set identity call

hello, I needed to access the value of lastIdentity to send the id in a url, i.e.:

http://www.mysite.com/api/v1/example?unique_id=123ABC

so I moved the property declaration to the .h file.

Also, I wanted to be able to call the identity method with a value and NOT have it really make an http call to the kiss metrics api if the value I pass in is already what lastIdentity is set to.  i.e. don't make kiss metrics report 'bob has been alias to bob' which it does if I call identity(@"bob") and it was already bob.  thanks!