first run at cellular connection type info

Based on:
https://docs.google.com/spreadsheets/d/16csKPy0dC2Hs6OHbLdWf0qlDiiRreqPtsMEE8vebRYo/edit
fixup cellular data

- dropping type
- moved ref into own struct with url and explanatory note
- added LTE advanced
update ceiling values

- switch to Mbps to be consistent with spec
- split ceiling into downlink and uplink
- uplink values are TBD
Merge pull request #14 from igrigorik/bandwidthCeiling

Connection type JSON struct