Add accounts

Accounts need an account_id (I'm imagining that I'll use the account_id field
from the OFX file here) that's both present and unique.

Transactions aren't currently linked to accounts.
Associate transactions with accounts

*NOTE* After running the migration in this commit, you'll want to update the
account_id of the account created, or manually merge the accounts (in the
Rails console) after a subsequent import.

* The migration creates a generic account and associates all existing
transactions with this account.

* The `StatementImporter` is responsible for finding or creating the account
referred to by the ACCTID field in the OFX file. It's possible I'll want to
restrict this further by BANKID in future but the ACCTID should be good enough
for now.

* Although accounts aren't referred to anywhere in the frontend, it *should*
(as of this commit) be possible to upload transactions from multiple banks and
not lose any data.
Allow accounts to be named

The `Account#name` defaults to `Account#account_id` when not explicitly set.
This isn't used in the frontend yet but I'm planning to add it to the table of
transactions soon.
Display the account name against the transactions
Merge branch 'add-accounts'

This is a little clunky but should hopefully be good enough for now. You can
upload statements from multiple accounts and they'll be associated with the
correct account automatically. The account name (which is only editable
through the console) is currently shown against each transaction but I imagine
that it'll make more sense to have the transactions index be nested under an
account resource in the near future.