Fix template bug for non-existance form field

When there is only one unpaid source account, the `source_type` field is
not added to the top-up form.  And when this field is passed to the
`form_field.html` partial, the `annotate_field` template tag chokes.

Fixes #6
Correct typos in README
Add flake8 to requirements
Reorganise makefile
Reorganise requirements.txt
Remove unnecessary TEMPLATE_DIRS entry
Bump version to 0.1.2