patreon: Exclude tax from full amount.

This makes the balancing logic in entry generation work the way we want.

patreon: Update importer for this month's change.

Literally all they did was change the title of the amount column
(maybe it doesn't assume USD anymore?).

patreon: Update importer for recent patron changes. RT#12747

All they did was change a couple of column names.

hooks.ledger_entry: Clean up whitespace in strings.

The hook now ensures it does not output whitespace that could be
significant to Ledger, either because it's a newline or an
account-amount separator.

hooks.ledger_entry: Improve date handling.

Check that we have the date field used in the payee line, and not just
'date'.
Allow other date fields to be None since they may not be used by the
template.

nbpy2017: Report item rate names and final unit prices.

This keeps more of the RBI calculation in the templates, where it belongs.
Template logic has become more capable since this importer was first
written, which makes this change practical.

csv: Support importing squared CSV spreadsheets.

See the test comment for more rationale.

ledger_entry: Only try to balance entry amounts when necessary.

This gives template authors the ability to write entries that
balance something other than the "main" amount.

importers: Catch and handle CSV parse errors in can_import.

import2ledger bailed on one of these exceptions when trying to import
a recent XLS file that had a null byte on a line.

import2ledger: First version.

This is a pretty feature-complete version 1.
I don't know why I waited this long to commit anything.