Fixed all issues in #228, thus:

-Removed "use namespace std" everywhere (not just headers)
-Abandoned naming "_name" in favor of "this->name"
-Dropped "vector" overload of "validate"
-Dropped "all" getter from "Results"
-Added "getWarnings" & "getErrors" from "Results"
-Changed the way prefixes are added to msgs: not in the ctor anymore but only upon output by "<<"
-Dropped all overloads in checks except for one "bool operator() const" returning implicit bool conversion or "val.empty()" boolean
-Changed the way the call to validate is written: now all is written inline
-Made the headers of conditions more consistent: all is "const &" now, though this is nonsense for the pointer-to-member