Fix Cursor Issue When Using True Color

This fixes an issue where, when using true color, the changeCursor
function would not actually write the cursor to the target array.

Fixes #407
Added in guidelines for contributing to noVNC

In CONTRIBUTING.md, you can now find a set of guidelines for
contributing to the noVNC project.  They detail coding style
information, requirements for pull requests, and how to run
the unit tests.
Merge pull request #410 from kanaka/docs/contributing-guidelines

Added in guidelines for contributing to noVNC
Modify minimum width of clear to accomodate smaller screens
We should use the globally declared UI variable
Fix broken tests from changing default screen size

Commit 795fca23dc6cf124b2e6969ee42bd8e7c836d605 changed the default
size from 640 to 240.  This broke a couple tests which depended on
the default size being 640.  Those tests have now been fixed.
Fix Karma sinon-chai version issues

Previously, we were using the karma-sinon-chai package to
provide sinon-chai to karma.  This used an older version of
sinon-chai, and looks to be no longer maintained (it's been
a month since sinon-chai was updated).  A new package,
karma-sinon-chai-latest, is now used.  This package uses the
latest version of sinon-chai and sinon, just like karma-chai
uses the latest version of chai.
Don't draw "blank" HEXTILE tiles with random data

Previously, if a HEXTILE tiles was received with a subencoding
of 0x00, it would draw a rectangle using data from the render
queue, which would result in random colored blocks when using
the HEXTILE encoding.  This is the result of a miscopy during
the refactoring.  It now has the correct functionality according
to the RFB protocol specification, which is to draw a rectangle
with the last set background color.

Closes #411
Merge "pull-request/412" (ramondeklein/noVNC)

Closes #412