Removed unnecessary standard filesystem library. (#726)

* {src/common/http_header.cc,include/pistache/http_header.h}: Implemented support for Authorization headers using basic method...
include/pistache/base64.{cc,h}: Added utility classes and helper functions for base 64 encoding and decoding...
tests/headers_test.cc: Added unit test authorization_basic_test...
version.txt: Bumped versioning metadata...

* Reformatted last commit with clang-format(1) per Dennis' request. Looks awful.

* include/pistache/base64.h: No need to include standard filesystem library...

include/pistache/base64.h

@@ -14,7 +14,6 @@
 
 // Standard C++ / POSIX system headers...
 #include <cstddef>
-#include <filesystem>
 #include <string>
 #include <vector>