Add first test code to build images. Surveillance fails.

- Images built for previous days only.
- Exception in process_previous_days_thread.
Fix bugs found in basic previous-days test.

- Fix exception in make_index_page() trying to generate link to
  previous page when on oldest page.  Now generates grayed
  "Previous day" pointer.
- Fix bugs and exception in process_previous_days().
- Clean up comments in main().
- Test results validated by eye, not yet by test code
Checkpoint. Partial implmemtation of website validation.
Complete implementation of validateWebsite().
Improve testability. Add new tests.

- Add daydirs_with_work() to surveillance to filter out daydirs that
have no images to be processed in main loop.
- Add terminate_processtoday_loop flag to return from processtoday()
when all of today's images have been processed during testing.
- Modify setUp() in TestSurveillance to remove any existing log file
before the start of a test run, and reset all test globals in
surveillance.
- Change copied and pasted test function & global names from
ftp_upload to reference surveillance.
- Change all references to surveillance module to moduleUnderTest.
- Add two new tests; still need proper code to set "today".
Add log messages. Fix bugs in tests.

surveillance.py:
- Add log messages to processtoday().
- Remove newlines from all log messages.
TestSurveillance.py:
- Add ForceDate function.
- Fix minor bugs in tests.
Add purge testing.

surveillance.py:
- Add files_to_purge flag for testing.
TestSurveillance.py:
- Improve validateWebsite().
- Insure each test sets today's date.
- Add purge test.