(doc) add a section about the new 'erlang-default' alias property
Update piqi_binary dependency to v0.6.5

Also, update (reboot) src/piqi_piqi.{erl,hrl} to the latest (v0.6.5) self-spec
that has support for the new piqi/file and field/json-omit-missing properties.
Add travis-ci build status image
piqi-erlang v0.7.0 release