Adding boost::is_complex to type_traits.hpp

[ejgrace]

Following a discussion on boost-users I am adding this
to the bug system so it doesn't get lost.  

Can someone add a boost::is_complex<T> trait so that
checks for complex numbers can be made.  This is a
common requirement in templated scientific code as
complex numbers sometimes need to be treated differently.

Attached is my implementation, please feel free to
incorperate it.