Add is_blank() string functor classification

[glywk]

In string algorithm, is_blank standard classification exists since C++11. Today, this classification is not provided by this library for code.
I have added is_blank functor in the implementation compiled with standard upper or equals to c++11 and complete the documentation.

[glywk]

@mclow

[glywk]

@mclow as suggested, I have add a negative is_blank() predicate test case. It checks that:

• first, any of chars are not blanks. '\n','\v','\f','\r' are classified as spaces characters as describe in std::isblank
• second, not all chars are blanks. (ie. 'a','x')

[glywk]

Is something miss?