This commit makes two BREAKING CHANGES to the file cache format!
- There is now an integer `modtime` column in the SQLite database. This
  is easier than using the filesystem modification time since we already
  need to query the database for the ETag.
- There is no longer a .png extension on cached files. There's no reason
  to assume the file is a PNG image, so it was unnecessary.

Makes ShumateFileCache its own class, rather than a subclass of
ShumateTileCache. It is curretly unused, but in the near future it will
be directly used by ShumateNetworkTileSource to cache tiles.

As part of this change, ShumateFileCache no longer fills tiles; it has
asynchronous methods to store and retrieve tile data, but it treats this data
as opaque. It is up to the user of the file cache to decode tile data.

Part of #15.