Add CodeMirror

As discussed in issue #1540, what about adding some code highlighting to the HTML code?

This pull request adds it to the HTML block type. If the core team accepts this, we could switch to CodeMirror also for the "View Source" of the Content block type.

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Before:
![before](https://f.cloud.github.com/assets/928116/1734794/a5250d4e-634b-11e3-8c17-a44a5e3a17c3.png)
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After:
![after](https://f.cloud.github.com/assets/928116/1734797/ac9e233a-634b-11e3-9e9d-6455745b35cb.png)

**The new HTML block type also behaves better when the dialog is resized...**

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Before:
![before-resized](https://f.cloud.github.com/assets/928116/1734804/cdb0dbe4-634b-11e3-824c-afd8bab79e68.png)
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After:
![after-resized](https://f.cloud.github.com/assets/928116/1734807/d49eaa26-634b-11e3-8e83-e1371bd96ea3.png)