Add scroll cursor commands (z enter, zt, z., zz, z-, zb)

![dim5o](https://cloud.githubusercontent.com/assets/438664/4791379/881251e4-5ddc-11e4-9142-326d54c20298.gif)

Fixes #103. Included commands:
- [z enter](http://vimhelp.appspot.com/scroll.txt.html#z%3CCR%3E)
- [zt](http://vimhelp.appspot.com/scroll.txt.html#zt)
- [z.](http://vimhelp.appspot.com/scroll.txt.html#z.)
- [zz](http://vimhelp.appspot.com/scroll.txt.html#zz)
- [z-](http://vimhelp.appspot.com/scroll.txt.html#z-)
- [zb](http://vimhelp.appspot.com/scroll.txt.html#zb)


This is also on the Editor model, `@editor.getFirstVisibleScreenRow()`