bugfix: continue is a C++ keyword. refactoring enumeration value.

status_code::continue is status_code::continue_ now

following boost convetion (int_, char_, ...), but it can change later.
fix: usage of "C" types like size_t and uint8_t

- Make sure they're included.
- Make sure they're referenced correctly (std:: prefix when not imported
  to referencing environment).
fix: invalid memory access on handwritten loop

also refactored to use standard algorithm copy_n
refactor: using lambda to select tolower overload

It is more readable than the function-to-pointer version.
add reminder to use dynarray when C++14 is enabled
add new error condition: http_errc::buffer_exhausted
handling buffer exhausted condition

The user will get notified when the buffer gets exhausted.