Fix #3

Improve use of existing field names when defining history structures,
tweaked source locations in the docs, and finished implementing much
better tests for the history routines. The tests still aren't
comprehensive (but they probably never will be given the complexity of
options available), but the new tests now cover all routines and should
work even if they happen to run over a day boundary due to some cunning
data-change table queries.

During the clean, also discovered that changes views don't produce the
correct timestamp for deletion and update events. This is now corrected
and tested for
General tidying up

Fixed installation and removal so it works when the destination schema
doesn't pre-exist and fixed the drop-schema test to ensure it doesn't
clobber the CURRENT SCHEMA special register for the subsequent tests
Merge branch 'master' of github.com:waveform80/db2utils