Add bell's own logger
Switch to bell's logger
Upgrade node-ssdb and remove bunyan deps
Remove useless util `startsWith`
Dependencies versions:
1. co upgrade to 4.0
2. fivebeans extactly to 1.1.1
3. koa to latest version: 0.13
Let bell works with co@v4 with bluebird
Merge branch 'dev'

* dev:
  Let bell works with co@v4 with bluebird
  Dependencies versions: 1. co upgrade to 4.0 2. fivebeans extactly to 1.1.1 3. koa to latest version: 0.13
  Remove useless util `startsWith`
  Upgrade node-ssdb and remove bunyan deps
  Switch to bell's logger
  Add bell's own logger