GH-120 Documentation updates

  Standardize on 'R10k' as the proper noun in all *.mkd files.
  README.mkd:
    Updated documentation links
  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.
  doc/dynamic-environments/workflow-guide.mkd:
    Completed workflow guide.
  doc/puppetfile.mkd:
    Link to description of the Control Repo
  docs/best-practices:
    Created an initial draft of a best practices guide for users to follow.
If a forge module has a version of `:latest` it will work; it would be better to link to the puppetfile documentation to explain the semantics of different module types rather than call out special cases.