Make block cursor cover character underneath it

The block cursor does not completely cover characters that are wider than a non-breaking space (the width is set at one single "&nbsp;"), such as CJK characters.

This change updates the `CursorView` with the character under the cursor whenever it moves (i.e., "`cursor:moved`").

(from atom/vim-mode#187)

 *  CJK Character
     *  Before: ![image](https://f.cloud.github.com/assets/189989/2460955/d4844204-af71-11e3-9a48-da85b2474e39.png)
     *  After: ![image](https://f.cloud.github.com/assets/189989/2460996/390aa7fe-af72-11e3-9ac4-3ffe93523c3e.png)

 *  Tab
     *  Before: ![image](https://f.cloud.github.com/assets/189989/2461189/6e0b44ca-af74-11e3-8f77-5a3db21c7dcc.png)
     *  After: ![image](https://f.cloud.github.com/assets/189989/2461183/56305ce6-af74-11e3-985a-ddc437efae2a.png)

I still have to figure out how to write the specs for this, though...