Fix compilation on Xcode 5.0.2

This fixes Issue #92. In particular, 3 changes were required:

 * include `<locale>` instead of `<ios>` to get `std::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 in this regard).

 * include `<iosfwd>` in `bsls_iosfwd.h` instead of attempting to forward declare `std::char_traits`.  The latter proves difficult 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.