Properly disable presentation mode when the viewer is embedded

Currently presentation mode isn't properly disabled when the viewer is embedded in a web page.
__Steps to reproduce:__
1. Open http://people.mozilla.com/~ydelendik/presentations/ecma5.html#/2.
2. Click anywhere within the PDF.js user interface.
3. Enter presentation mode, by using the keyboard shortcut `Ctrl+Alt+P`. ~~(Note that the toolbar button isn't visible, but that's only because the PDF.js UI is too small.)~~ *EDIT: Now that the secondary toolbar has landed, the presentation mode button will be available from it, thus making it a lot easier for the user to enter a non-working presentation mode.*

__Expected result:__ Presentation mode should either work, or be disabled.

__Actual result:__ (A picture is worth a thousand words...)
![presentationmode_embedded](https://f.cloud.github.com/assets/2692120/1079656/688d892a-154d-11e3-945a-20d598c6fb73.png)

This PR adds a check in `PDFView.supportsFullscreen` to disable presentation mode when the viewer is embedded, to avoid the above issue.