Update README and sample app

* uses Hash rather than individual parameters
* puts :host rather than :hostname in YAML file
(whitespace and reformatting)
Relax RSpec dependency; avoid nil dereference

* Change from ~> 2.13.0 to just ~> 2.13
* MktoRest#authenticated? shouldn't fail when @token is nil
Improve Lead#to_s

Use .inspect so that the output is more likely to be
copyable and remain syntactically valid.
Allow :open_timeout and :read_timeout as options

* Pass these options through to later MktoRest::HttpUtils calls