added is_convertible and alignment_of

svn path=/branches/conversion/boost/boost/detail/ob_type_traits.hpp; revision=7664
preview of updated version with is_convertible and alignment_of added.

svn path=/branches/conversion/boost/boost/detail/ob_type_traits.hpp; revision=7667
type traist update (first release)

svn path=/branches/conversion/boost/boost/detail/type_traits.hpp; revision=7673