Started implementing time entry extraction.

Time entries are now being parsed and validated, though there are
numerous issues still to sort out.

I have a feeling that further development will require passing around
the `top` style attributes in the same way I'm passing around the `left`
style attributes. TBD though.

Significant updates

Wrote time sheet parser that ingests and validates all semi-structured
data. Next step is to interpret left styles as dates, so that hours can
be parsed into a time entry object.

Updated HTML parser to more completely filter out unhelpful data, and to
internally build the array of doubles (data and left style).