Allow prevention of PhantomJS download

* Allow users to purposefully avoid automatic download and installation
of another PhantomJS
* The download and installation is over http and without a checksum,
which poses a security risk
* When preventing download, PhantomJS already on the path is used
* Maintains current default behaviour of downloading if unavailable
* Include example in the generated jasmine helper
* Update readme to mention option
Merge pull request #213 from fofr/preventable-phantomjs-install

Allow prevention of automatic PhantomJS download