Saving with `pathlib` object

[williambdean]

Would be nice if the save functionality worked with pathlib's PosixPath object.

No python requirements would have to change with pathlib being introduced in python 3.4

Seems like check would go here:

branca/branca/element.py

Lines 162 to 163 in 629ed34

	if isinstance(outfile, str) or isinstance(outfile, bytes):
	fid = open(outfile, 'wb')

[Conengmo]

That seems like a simple change, so I'd say a PR is welcome.