Fix implicit conversion changes signedness problems.

clang-3.5 trunk 201163 and OS X's clang (not sure which version)
report -Wsign-conversion warnings with this header. This patch fixes
them.

https://svn.boost.org/trac/boost/ticket/9725

Signed-off-by: Thomas Klausner <tk@giga.or.at>
Merge pull request #6 from 0-wiz-0/master

Fix implicit conversion changes signedness problems.

Looks good, thanks!