Update project config files to work with Grunt 0.4.
Update wkthmltopdf task to use Grunt 0.4 multi-task API.
Update a lingering reference to Grunt 0.3.
Improve predictability of the destination for created PDFs.

Now generated PDFs are placed at the destination specified in the
end-users's Gruntfile, rather than appending the source file's path to
the destination path. The created PDFs have the same name as the source
HTML file, excep the file extension is changed to .pdf.
Merge pull request #8 from starim/grunt-0.4

Update grunt-wkhtmltopdf with work with Grunt 0.4