ofx: vendor parse_ofx_time implementation

[jktomer]

The ofx source makes use of a simple function to parse the OFX time format from an OFX importer that used to be included in Beancount. Unfortunately that importer has been demoted to a separate repository beangulp, and to the examples directory in that repository to boot. This change therefore copies the (trivial) implementation of parse_ofx_time from the beangulp examples into ofx.py.