Closes #4: Use phantomjs to provide a reliable path to 'phantomjs'.

To support this, The 'phantomjs' node package from was moved being an
optional to hard depedency.

It appears the 'phantomjs' package will detect if phantom is already installed, so
it doesn't inefficiently install a second copy.

Manually tested by with example.js.
Merge pull request #6 from markstos/phantomjs-path-issues

Closes #4: Use phantomjs to provide a reliable path to 'phantomjs'.