Previously we took the address of the dereferenced begin() iterator
on a string to get the pointer to its data; we now just const_cast
the pointer that is the result of calling data().
The original reason for using the iterator over something like data()
was to avoid the const_cast and to ensure that any copy on write
mechanisms were used, but this doesn't seem necessary given that we
call resize() immediately prior to this. Valgrind shows no problems
with the new method. Also added unit tests for this case to string.
closes#38
Added a preprocessor define CEREAL_OLDER_GCC that exists if using
any GCC 4.7.x or earlier. If this is defined some type traits change to
our original solution (prior to VS compatible), which is probably what we'd switch
to if VS ever gets around to fully supporting SFINAE and constexpr.
In map and set, as well as in unit tests, use insert instead of emplace for 4.7.x.
