expansions option, error code fixes

* use getAccount to get account, as authResult.account will be deprecated
* add an option that allows you to specify the fields that are expanded 
* fix to pass on err.status, or default to 500 if undefined
fix tests

* stub out authenticationResult.getAccount()
* unset env vars for tests that are testing that case :)
fix to option passing
add .travis.yml
documentation of expansions option
Merge pull request #7 from stormpath/expansion-options

use getAccount() after auth, new expansions option