GH-120 Documentation updates
  Standardize on 'R10k' as the proper noun in all doc/* files.
  doc/dynamic-environments/configuration.mkd:
    Move invalid_branches description.
    Add purgedirs description.
  doc/dynamic-environments/configuration.mkd:
    Update basedir to reference purgedirs.
  doc/dynamic-environments/git-environments.mkd:
    Clarify the details of how r10k works with git repositories.
    Update introduction to reflect environmentpath beginning with puppet v3.5.
  doc/dynamic-environments/quickstart.mkd:
    Provide alternate method to access a dynamic environment.
  doc/dynamic-environments/usage.mkd:
    Clarify how forge modules are treated.
  dynamic-environments/workflow-guide.mkd:
    Partial completion of a dynamic environments workflow guide.