Add "Create Web Page Item from Current Page" keyboard shortcut, followin...

...g the example of the "Save to Zotero" shortcut.  Address zotero/zotero#178

A few comments:

* I tried to follow the style of `key_saveToZotero`, so I think this should be easy to review.
* I was not sure what to choose for a good default key.  The best choices mnemonically are taken (`C` and `N` by Zotero, `W` and `P` by Firefox).  I went with `I` for now.
* I did not add a case to `handleKeyPress()` of ZoteroPane.js because `key_saveToZotero` does not currently have one.  So, like `key_saveToZotero`, `key_newItemFromPage` can not be called when Zotero is focused.  I was not sure if this was the intended behavior for `key_saveToZotero` or an oversight.