fix for scrolling with bound element; added onInvalid callback and altFormats option

We frequently have the need to use scrolling on container elements, and at the moment, pikaday stays stuck to its original position rather than following its bound field.

I did some poking around in the code and I found a couple of things that seem to work to allow this. I'm sure there are other considerations, but hopefully the general approach will work or will at least work for a specific use-case.

Summary of changes:
If NOT bound, append picker to document, otherwise if bound, then insert picker into DOM as sibling of its bound field.
Then adjust the picker position during draw to eliminate the top positioning and remove the extra left offset which should now be handled by the default document flow.