Remind users about breaking changes using post_install_message
unify the way we test
users are responsible for building a correct env by themselves, see:

we introduce Client#default_env which helps you create a default env.
Merge remote-tracking branch 'cblue/payload_post_install_message'

* cblue/payload_post_install_message:
  Remind users about breaking changes using post_install_message
add an extra period
add license and post_install_message to gemspec
em-http-request.rb: use RC::Payload instead of RestClient::Payload
em-http-request.rb: our payload would never be nil
em-http-request.rb: simply pass the file if payload is already a file
version bump to 2.1.0 and update CHANGES