Adds support for local installation of phantomjs. Fixes #31

I had some troubles using local installs with the latest version, as I see others have in issue #31.
This seem to be due to the pull request #29.

In CI-systems it makes sense to have local installations to not mix versions. 

In my case the `cmd` variable (`location` from PhantomJS) was 

```
somedir\node_modules\phantomjs\lib\phantom\phantomjs.exe
```

which caused the `phantom`-variable to be 

```
somedir\node_modules\phantomjs\lib\phantom\node_modules\phantomjs\bin\phantomjs
```

This is a non existing path, as the path really is without the `lib\phantom`.


I'm not entirely sure that this is correct for all instances and all cases, but by removing the `lib/phantom` it worked with both global and local installs on our system - with fresh installs. 

It would be great if e.g. @gillius could test this as well.