Cleans up internal structure of the code.

New directories description:
	Root, with CMakeLists that sets compiler stuff (warnings, usage of address-sanitizer, etc.)
		rusql, with the abstraction classses that rusql provides, such as rusql::Connection, and rusql::Database
			mysql, with the implementation of our wrapper around the mysql-c-api

	Next to mysql, there's place for more implementations, if we want them.