Implements first steps of bridge config.

1. Bridge checks for and creates the root configuration directory when
setting up a SteamAPIContext object (after Init is called).
2. Slightly better order to SteamAPIContext's object members.
3. Added 'disclaimer' bool, to track whether the user has accepted
the disclaimer.
Removed Boost dependancy because COMPILE TIMES.

1. Just removing the unused property_tree include reduces single process
complete compiles by nearly six seconds (19->13)!  Probably going to
use libconfig instead.