Add Public API metrics and violations

[lcintrat]

Add CxxPublicApiVisitor which counts documented and undocumented public API items.
Add UndocumentedApiCheck that creates issue for undocumented API.
Issue #307

[guwirth]

Hello @lcintrat, I think this is a very useful extension.

• Had a similar idea but like to use doxygen warnings to detect missing comments. Your solution has the advantage that no additional tool is needed; drawback is that parser must work accurate (seems currently not always be the case).
• Your design decision to say 'public API' must be in header files makes sense.
• Not sure if 'protected' should count as public API?
• In case of derived classes it is possible to derive comments in doxygen (\copydoc). Don't know if/how this is supported.

[lcintrat]

Thank you for your feedback @guwirth

• I agree with you, this extension depends on the parser. Doxygen can also detect errors in the comments (missing arguments, invalid tag, and so on). However, this extension gives a metric, which is very useful.
• IMHO 'protected' should be included in public API because a client can always derive a class and use protected methods, this is a normal case.
• Since the extension does not validate the comments (at least, it should check that they are not empty), if a derived class has methods with Doxygen comments (including \copydoc or not), this is counted as documented public API.
• As argued in Missing "public documented API" measure #307, the extension could also consider some predefined macro to detect public API (e.g. #define DLLEXPORT), but this is another achievement.