Added getCanvasContext utility function that allows specifying the host ...

...element as either a textual ID or DOM element object.  Replaced all getElementById calls with this function.

This resolves issue #8.

You can now create gauges on elements without having to use HTML IDs.  This will allow me to create Knockout databindings for the gauges (forthcoming).

NOTE: This commit does not contain the minified version, because I could not deduce which minifier you guys are using.  All of my attempts produced a min version that was much larger than the current.  If you could please specify the minifier and args you are using in the README then I'd be happy to handle that too.