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.