Extend with NFC support

This potentially fixes #37
- Add NFC permissions to manifest
- Add dependency on otp client library
- Change all activities to perform a task and return the result, rather
  than accessing TokenPersistence directly
- Add internal and external main activities managing internal and
  internal and external tokens
- Extend InternalMainActivity to open up ExternalMainActivity when an
  external token is presented
Merge pull request #16 from fidesmo/centralize-token-handling

Centralize token handling