Remove unnecessary require

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Promote TrivialAccessors exception

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Introduce bosh-core gem

* To extract reusable code from bosh-dev into a production-worthy location
* bosh_common isn't a good candidate as it doesn't respect the loadpath
  i.e. require 'common' vs require 'bosh/core'
* Too many things depend on bosh_common to change it safely

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>