Introduce custom keyboard shortcuts for SizeUp

Trigger [SizeUp][1] actions using keyboard shortcuts that are on the home row, or very close to it:

![keyremap4macbook](https://f.cloud.github.com/assets/2988/1144046/f7e166a6-1d6c-11e3-95e3-fbfce81e6401.png)

It helps to visualize these mappings:

![skitched-20130914-144505](https://f.cloud.github.com/assets/2988/1144075/9ae200bc-1d6e-11e3-9462-6e8e5226b6c3.png)
![skitched-20130914-145047](https://f.cloud.github.com/assets/2988/1144076/9d1a8e1c-1d6e-11e3-9f75-26ed8c14cadd.png)

---

TODO:

- [x] Define shortcuts in private.xml
- [ ] Update README

[1]: http://www.irradiatedsoftware.com/sizeup/