Implement loading config file from a default path

  Load `config/zassets.yaml' when the file exists, prior to loading
options from -c argument as `-c config_file.yaml' or other options
provided on the command line.
Add cucumber and aruba test dependencies
Add monkey patch to hack aruba steps (must, should)
Add monkey patch to fix cucumber docstrings:

  Cucumber chomp the last \n from multiline strings (docstrings), this
monkey patch modify Cucumber::Ast::DocString constructor to add the
missing ending new line character and
Cucumber::Formatter::Pretty#doc_string to remove it from the output.
Add user acceptance tests:

  Add many scenario testing the CLI, configuration, some server
behaviors, a few engines and how assets are built.

  Also add a testing dependency on httparty gem, which is used when
testing the server.