viewport improvements

* Get rid of Crafty.viewport._zoom which is synonymous with
Crafty.viewport._scale. There is no point in having two redundant
properties.

* Eliminate temporary-object-creation in Crafty.viewport.rect(), by
making a persistent object. (To avoid unwanted interactions, this object
should be read-only, which required me to edit diamondiso.js.)

* Improve documentation
Merge pull request #702 from sbyrnes321/develop

viewport improvements: eliminate redundant _zoom property, improve docs