Enlist all PHONY targets in Makefile
Don't start cowboy as part of piqi-rpc supervision tree

This fixes all kinds of nasty race conditions in stopping piqi-rpc.

Additionally, see [this](https://github.com/extend/cowboy/pull/265) issue on
why this is not supported and not the cowboy way of doing things.

Also added tests and refactored the `piqi_rpc_http:get_http_env` to not require
to always pass the default explicitly.
Move env maintenance using a gen_server / ETS

This a standard technique to maintain some stateful application configuration:
an ETS table owned by the supervisor + gen_server communicating with it.

This removes all the nasty hacks needed for cleaning the application env when
stoppin and guarantees that restarting the application will always give you a
fresh state.

Also, added some more tests.
Final touches on the start/stop problems

Figured out that the problem was a ranch bug.

Also, made sure piqi_rpc_http (i.e. cowboy) is always in sync with piqi_rpc
server even if it crashes and restarts.
Merge pull request #8 from yfyf/fix_cowboy_stop

Fix application:env and stoping/starting related issues