Support for Installing from Edit Page without Commit

When open a file to edit in GitHub, this will add a button saying checking if its installable. It will look at the file path of the currently open file and then look in its parent directory, then grandparent directory, and so on until it finds an `install.rdf`, when found, it will set the path on the button to this directory. Clicking the install button will download the zip and replace the currently loaded edit page file in the zip with the one from the sourcecode. Allowing to edit on GitHub and commit only when bugs are ironed out. Before this PR, if a user wanted to work out of the GitHub edit file page, they would have to commit every change and then install to test.

`nsIFile.remove`'s were also changed to use `OS.File.remove(nsIFile.path)`
I'm wondering what this line does...
I'm asking because this could conflict with other addons and userscripts.