This commit fixes 'bsl' on clang 5 (issue #92)

This fixes Issue #92. In particular, 3 changes were required:

 * include <locale> instead of <ios> to get 'num_get' in 'bsl_ios.h'

 * include <sys/time.h> instead of <time.h> to get platform-specific types related to time (making Darwin more like FreeBSD and AIX)

 * include <iosfwd> in 'bsls_iosfwd.h' instead of attempting to forward declare 'std::char_traits'.  This proves difficult with this version of clang since we cannot guarantee, in general, the ordering of 'bsl_iosfwd.h' and 'bsl_string.h'.  The latter would pull in <string> which would define it and clash with the definition provided in 'bsl_iosfwd.h'

I am aware there is an [ongoing review](https://review.openbloomberg.com/D56) for these fixes; I believe my pull request addresses concerns raised in that review.

Needless to say, all tests passed.