Configurable number of PhantomJS instances from the includer's package.j...

...son

After this commit, one who depends on attester in his "scripts" part of
`package.json`, can additionally put in that file the following:

     "config":{"attesterThreads":2}

to specify the number of PhantomJS instances (instead of passing it via
command line), but as the added benefit, the actual user who npm install'ed
his package, could type:

     npm config set somePackage:attesterThreads 8

to take advantage of his multi-core machine.

This setting is taken into account only if there was no
`--phantomjs-instances` passed from the command line when invoking attester.