Fixed conflict between Win32 headers and STL.

1. Apparently the win32 API defines min & max macros, which can conflict
randomly with other libraries.  Fortunately, there's a macro that
disables them.
Considerable fixes to bridge callbacks.

1. Updated checks/messages considerably.
2. The callback base class now sets flags/callback to 0.
3. After researching the behavior of CCallbackBase, changed how/when
things are set.
4. Added some documentation about the behavior of Callbacks.
Added some basic steam_api testing code.

1. Created a directory (testing) that holds a simple Linux app that can be
used for poking the behavior of the Linux steam_api.so library.  This code
can be more readily testing in valgrind/debuggers/etc than through Wine.
Documenting/cleaning of bridge's callback.

1. Tweaked various documentation comments in callback.
2. Removed some old commented out stuff and whatnot as well.