Improve conversion crud (get data from Wolfram Alpha)

[Igor Santos]

1. Get index from Wolfram|Alpha:
   • http://products.wolframalpha.com/api/explorer.html
   • https://developer.wolframalpha.com/portal/apisignup.html
   • http://think2loud.com/224-reading-xml-with-jquery/
2. Disable unusable units (those from different groups or the one selected in the other box)

[Former user]

• adding a simple trait to enable easier creation of a JSON API controller. refs #11

→ <<cset faee4dc167a0>>

[Former user]

• adding basic support to Wolfram|Alpha API, including styles for a button to call it. refs #11

→ <<cset fdaf4c1a6a3d>>

[Former user]

• implementing getting automatic value for unit conversion from Wolfram|Alpha. refs #11

→ <<cset f6e3a2fcba01>>