Checkin comment optional.

The checkin comment should be optional for a user, therefore gever should provide the two following actions:
* Checkin with comment
 * Process just as before
* Checkin:
 * The comment form is not shown, the document will be directly checked in. The redirect should be the same.

Actions display:
1)![image](https://cloud.githubusercontent.com/assets/485755/3223073/72fe72be-f01e-11e3-8af0-03297ff13f18.png)
2)
![image](https://cloud.githubusercontent.com/assets/485755/3223076/755f2210-f01e-11e3-9ec4-76d0014cb82b.png)



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When using the `plone.api` it would be much easier to read..