Moved pure_traversal_tag to public namespace.

Also added an import into the boost::detail namespace for backward compatibility with Boost.Range. Added a pure_iterator_traversal metafunction that automatically converts iterator category as well.
Copied minimum_category.hpp to public headers.
Moved minimum_category to the public namespace. Added tests.
Adjustments for compatibility with commit https://github.com/boostorg/range/commit/48dfb6804598a1890198b0ecde48e4a0d1812ea1.
Merge pull request #6 from Lastique/publish-details

Publish some of the implementation details

Tested with clang-5.1 on darwin.