WADIndexer: added new method index_wad()

- index_wad() indexes one WAD file
- Renamed existing method index() to index_wad_list(), which
  enumerates over a list of WAD files, and calls index_wad() on each
  file
WADFile: added num_of_lumps and wad_signature attributes
wadindex: call index on each WAD in a zip

- Instead of calling index() on a bunch of WADs in a list, call
  index_wad() on a single WAD, which should return a WADFile object
- Moved the checksumming of the Zip file to be right after the ZipFile
  object is created
- Don't pass the index database to the indexer anymore, the indexer
  will return a WADFile object, which will then be passed to the
  IndexDB so it can be inserted into the database
- Added a bunch of FIXMEs for adding timers