Add customization macros for global new/delete

As mentioned in #181, some environments may require adaptations to
the internal calls to the global `new`/`delete` operators, like
adding explicit `NULL` checks to `delete.

This patch adds two new macros
 * RAPIDJSON_NEW(x)
 * RAPIDJSON_DELETE(x)
to allow user-defined expressions in these cases.

This fixes #181 in an alternative manner.
Merge pull request #183 from pah/feature/wrapped-new-delete

Add customization macros for global new/delete