Adds a readonly option to the SqliteDb. 

Sometimes you only want to read. This adds a readonly flag to the SqliteDb constructor and open() function.

This isn't urgent, but sometimes will reduce our memory problems by never creating a journal file for a db that is read only (http://web.utk.edu/~jplyon/sqlite/SQLite_optimization_FAQ.html#transactions). It also, clearly, protects files that we don't expect to write to.

I implemented it to protect a table I'm using Cyder, but have here implemented the flag to do the same protecting for the mass table when we open it for reading.