Major re-write of Stem
Still porting over the core from v0.17

Added the following handlers:

- tradeProposed
- offerChanged
- sessionStart
- webSession
- tradeError
- tradeMessage
- tradeReady
- tradeEnd
Finished porting over core from 0.17

- Added inventory caching
- Added command: items reload
- Simple error handling when bot inventory is empty
Added winston logging again

- Added more error handling
- Various fixes throughout
Fixed inventory cache not being reloaded after a trade
Merge branch 'beta'