coverage excluded from make all - use make coverage instead
code style changes
remove unnecessary include util from static object
adding nicer EnableIf and DisableIf
convert output XML archive to EnableIf
add inline to functions changed in prior commit
Using default template paramter for Enable/DisableIf

clang can't handle a variadic template pack for SFINAE overloads with
the new EnableIf/DisableIf, so we have to use a more verbose version
that uses a default template parameter (cereal::traits::sfinae).

see #85