Refactor HTTP client and update automated tests.

[cmpaul]

This closes issue #13 .

Potentially breaking changes:

• The following now return boolean instead of int:
  • HelloSignClient.cancelSignatureRequest()
  • HelloSignClient.destroyTeam()

Other, non-breaking changes:

• Refactored HTTP client code to be testable.
• Deprecating all HelloSignClient.getOauthData methods that do not send a state parameter.
• Rebuilt JUnit tests to run off Mockito using pre-generated responses.
• Added travis-ci config to run tests against all changes.
• Reformatted entire code base.

[cmpaul]

Notes from review:

• Bump to 4.0.0 due to breaking changes listed.
• Remove methods for getOauthData that don't include a state parameter
• Update DEFAULT_OAUTH_TOKEN_URL to app.*
• Make HttpClient injectable through the protected constructor, update tests
• Remove HelloSignClient(String email, String password)
• Remove HelloSignClient(Authentication auth)
• Don't swallow exception in isAccountValid()
• Remove createAccount(String email, String password)
• Remove createAccount(String email, String password, String clientId, String clientSecret)