Added function to calculate length of UTF8 string.

 * Add function `utf8_strlen`. Function counts number of symbols
   in the given string.
 * Add example.
Added `utf8_strlen_sse2` function.

 * Add `utf8_strlen_sse2` function.
 * Add BUILTIN_xxx macroes.
Updated strings examples.

 * Use `utf8_strlen_sse2` function.
 * Add include guard to the `utf8.h` header file.