Add strToDate function from Zotero framework

Integrate and adapt `strToDate` utility function from the Zotero
framework to consistently parse entries’ date fields cross browser. Add
it to our zotero_api global object and adapt zapi.js to accommodate the
extra function.
Adapt grunt tasks to include new strToDate file

Add grunt-contrib-concat, adapt debug task to use concat’s
`uglify_alternative` instead of copy, update our node modules, bump the
version of our package
Latest compiled version of zapi + sourcemap