TFJ-762 add Application-only authentication feature.

- Added files
  - twitter4j-core/src/main/java/twitter4j/auth
     - OAuth2Support.java (Interface)
     - OAuth2Authorization.java (Impl. of OAuth2Support interface)
     - OAuth2Token.java (Class)
  - twitter4j-core/src/test/java/twitter4j/auth
     - ApplicationOnlyAuthTest.java (Test cases of Application-only authentication)
- Added configurations
  - ```oauth2.tokenType``` ... Specify 'bearer'.
  - ```oauth2.accessToken``` ... Specify obtained OAuth 2 Bearer token.
  - ```oauth2.tokenURL``` ... Specify endpoint of obtaining OAuth 2 Bearer token.
  - ```oauth2.invalidateTokenURL``` ... Specify endpoint of revoking OAuth 2 Bearer token.
  - ```enableApplicationOnlyAuth``` ... Specify ```true``` if you want to use Application-only authentication.