Tied more closely SBuf and MemBlob size_type

Tied more closely SBuf and MemBlob size_type
Better compliance with standards
Improved SBuf documentation
Improved adherence to std::string method signatures
Improved \0-cleanliness
Removed some methods taking std::string arguments to avoid double data copies,
  documented alternate patterns
Implemented cmp and caseCmp shortcuts
Cleaned-up some methods' implementations after Alex Rousskov's suggestions
Improved unit tests' cleanliness by making more effective use of cppunit macros
Implemented more unit tests, better coverage for \0-cleanliness