Merged from trunk -r15210:15214

   Merged from trunk -r15210:15214

   | 15211   | Add support for evaluation of conditional USE atoms in       |
   | zmedico | has_version and best_version arguments, using the USE        |
   |         | environment variable.                                        |

   | 15212   | Add an Atom.evaluate_conditionals() method and use where     |
   | zmedico | appropriate.                                                 |

   | 15213   | Fix typo in docstring.                                       |
   | zmedico |                                                              |

   | 15214   | Fix AttributeError from has_versions for atoms with no USE   |
   | zmedico | deps. Thanks to Arfrever for reporting.                      |

svn path=/main/branches/prefix/; revision=15215