Minor: credits added.
Added: right mouse button (RMB) may act as activation key.
Add: ThumbnailZoomPlusOptions check options compatibility during init

New method _checkPrefsCompatibility() checks minVersion, maxVersion,
targetApplication and targetPlatform on per element basis.
Change: RMB option for activatekey pref is version and platform dependant

The "thumbnailzoomplus-options-activate-key-pref" preference element will
be disabled unless platform and Firefox version match requirements configured
in xul: Firefox 15.0

We use xpath rather than querySelector* because of compatibility:
document.querySelector and document.querySelectorAll are available since
Fx3.5. The document.evaluate is available since Fx1.5 and is the engine of
the new methods:
+ new method _xpathSelector replacing document.querySelector
+ new method _xpathSelectorAll replacing document.querySelectorAll

+ minor doc comment change
Prevent context menu to appear when TZP has made use of right mouse button
Added for RMB: handling of contextmenu events that happens before mouseup.

Important for Linux (and possibly Mac OS) compatibility.

- postponing contextmenu events after mouseup event, because Linux fires
  contextmenu event after mousedown.
- moved RMB specific code from _isKeyActive to _handleMouseMove, because
  _isKeyActive should not contain any additional logic.
- added some util functions.
- some code shuffling and refactoring.
Merge branch 'right-mouse-button'

Fixes #144: Let right mouse button to be activation key

Work by Vlastimil Ovčáčík; thanks for the contribution!
Commits reorganized/rebased by David Adler.
Renamed RMB to Right Mouse Button & added locale support

The Right Mouse Button label is now language-specific (comes from
locale.dtd.  But all languages' definitions are currently
English.