Finalize appSQL; document py src; modify logout.

Finalized appSQL.py:
	+ username availability check on registration
	+ change logout functionality: originally, logout was devoted
	an individual page, which, upon access, popped "loggedIn" from the
	flask session and redirected to "index". It's more logical to simply
	implement a logout button that submits a POST request, with a
	conditional in "index()"to pop "loggedIn".

Modify index.html:
	+ convert logout hyperlink to button to reflect logout functionality

Document appSQL.py/appShelf.py:
	+ add some documentation to both source files