Restructure save

Hi Boone-

Here's a first pass at moving the `$_POST` data requirements out of `BP_Docs_Query->save()`. I've added a wrapper function to get the `$_POST` data into the format that the save method expects.

A nice benefit is that it is now possible to create new docs programmatically by passing an arguments array to the save method. I'm sure I've missed some references to the current user or the current doc that we'll still need to deal with.

Please let me know if this is on the track you were hoping and what else I can do.