Create StatementStore to centralize handling of statements.

Closes #45

LGTM given by: @mattmanning, @rmcafee

Squashed commit of the following:

commit 793a9b5f51134113917e5a426c534a4f89123be3
Author: Kenny Parnell <k.parnell@gmail.com>
Date:   Tue Nov 11 10:15:21 2014 -0500

    Rename InvoiceStore to StatementStore :-/

commit 0f2ec57ef1d01dc929785da2a2bbbe5d0d7dea78
Author: Kenny Parnell <k.parnell@gmail.com>
Date:   Tue Nov 11 10:07:40 2014 -0500

    Add InvoiceStore.