Provide convenience methods for inventory creation
Added support for preliminary items

Also fixes a problem, where a GameInventory would not contain all
items.

Fixes koraktor/steam-condenser#187
Improved RCON connection handling once again
Fixed RCONSocket test
Added test for dropped RCON connection
Throw RCONNoAuthException when connection was dropped
Version bump to 1.3.1
Merge branch '1.3-stable'