Add support for gcc 4.6.
Add support for clang 3.3.
Merge remote-tracking branch 'LEW21/master'

Conflicts:
	README.md
	optional.hpp
Added .gitignore
Enabled some more pedantic warnings.
Removed constexpr_addressof().

It used old style cast (which is bad), which was in fact
reinterpret_cast, which isn't constexpr at all. When searching for
better solution on StackOverflow I found out that the proper solution is
already implemented here and this is just a leftover which is not used
anywhere anymore.