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)

nit: can we rename this to `CredentialsValidator`? I keep thinking this is a test class because of the name.