Fix the T4MVC errors I introduced.

I was doing merges yesterday on my laptop, which doesn't play nicely with
the T4MVC bits at all. This commit resolves the issues I inadvertantly
introduced.
Fix #101: make new Tweet button open a new window.

This particular fix uses target="_blank", which isn't great as it just
opens a new tab but it might be sufficient.
Merge pull request #105 from alastairs/issues/101

Fix #101: Make new Tweet button open in a new window