added tracker object methods

Hello Andrew,

I had a need for some additional tracker methods, and I saw you had a note that said you needed more options.  I added this functionality.  The input uses a hash of method names with corresponding values.  All three types of object arguments are supported (String, Boolean, and no arguments).  I also updated the Readme with the new functionality.

I added a test, which passes, but I was getting an failure on the custom variables test as it was checked out.  It can't seem to find the session variable in the tests.  I chalked this up to a test issue and I didn't resolve it.  I'm a bit of a noob and it would probably take me forever to figure out.

I also removed the "_trackPageLoadTime()" from the generated javascript, as Google says the method is deprecated.

This is only my second pull request ever, so feel free to tear me up.  Like I said, I'm a bit of a noob.

Thanks,
-Joe