Introduce custom keyboard shortcuts for SizeUp

Trigger [SizeUp][1] actions using keyboard shortcuts that are on the home row, or very close to it.

`Control+s` serves as the prefix for these actions. So, to trigger a SizeUp action, hit `Control+s` followed by any of the these keys:

![](https://f.cloud.github.com/assets/2988/1145223/f7764224-1dfb-11e3-813a-797eb6cc435f.png)

![](https://f.cloud.github.com/assets/2988/1145219/a8e4e930-1dfb-11e3-9092-f89fc4108e50.png)

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This Pull Request brings these shortcuts to KeyRemap4MacBook.

![keyremap4macbook](https://f.cloud.github.com/assets/2988/1145333/ccc6ffc8-1e0a-11e3-9b08-7e6e6b96a5aa.png)

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TODO:

- [x] Define shortcuts in private.xml
- [x] Update README

[1]: http://www.irradiatedsoftware.com/sizeup/