Fix composer issues

Your requirements could not be resolved to an installable set of packages.

  Problem 1
      - The requested package phpcollection/phpcollection == 0.2.9999999.9999999-dev could not be found.
        Problem 2
      - The requested package phpspec/prophecy == 1.0.9999999.9999999-dev could not be found.
Merge pull request #3 from jshedde/master

Fix composer issues
Add more details in the README file
Fix the returned type of the parse method
Parse sitemaps values with full urls correctly