Component API generator enablers

New features

• Initial version of component API generation based on JSON metadata about a web component
  • Still missing automatic JSON generation from the analyzer
• Updated Atmosphere to 2.4.5.vaadin2 to keep in sync with Vaadin Framework 8.x

Fixes

• Fixed some cases where the effective server-side and client-side model values could become out of sync

All changes

https://github.com/vaadin/flow/milestone/35?closed=1

Also all thanks to @gerald24 for the first external contribution done to Flow :-)