Various C++11-related changes.

* Switched to standard libraries for regex, unordered_map and
unordered_set.
* Switched to using the C++ syntax for C header includes (eg. `cstdint`
rather than `stdint.h`).
* Replaced IntToString with STL's to_string.
More C++11 changes.

* Made a few standard library replacements missed in the API.
* Swapped in the range syntax with the auto keyword in for loops where
appropriate.
NULL -> nullptr switch.