Rewrote ext to support multiple instances of Jasmine in one MM site

- Jasmine can be instantiated multiple times in one Middleman site/app
  by activating the extension multiple times in Middleman's config.rb.
- The __boot__, __spec__, and __jasmine__ URI paths for each Jasmine
  instance are contained within the instance's jasmine_url.
- JasmineSprocketsProxy has been replaced with separate app instances
  for Jasmine and Sprockets for each Jasmine instance in the app.
- The Sprockets instance is mapped to <jasmine_url>/__spec__, while the
  Jasmine instance is mapped to <jasmine_url>, responding to everything
  under that URI which is not part of the __spec__ path.
- src files are now under the Middleman app root, served by Middleman's
  sprockets app, so we no longer need to duplicate sprockets paths to
  our own Sprockets instance.
- Previously used proxy objects and PATH_INFO-manipulation logic are no
  longer needed and have been removed.
- Added option jasmine_config_path to specify the location of Jasmine's
  YAML config file (if not specified, puses Jasmine default).  This is
  used to configure each Jasmine instance with a different config file.