Implement OKPayCredentialsStepPane

**Description:** implement OKPayCredentialsStepPane and add some support code for it.
**Testing:**
![image](https://cloud.githubusercontent.com/assets/259587/3259770/dc8d58d8-f246-11e3-901e-15501af164ee.png)
![image](https://cloud.githubusercontent.com/assets/259587/3259775/e1138cec-f246-11e3-99f4-bad22d6644b9.png)

GUI feedback: it seems weird that the user has to "Test" the credentials. Shouldn't that happen when clicking the "Next" button? Alternatively, the "Next" button could be disabled until a successful test is run.