Added support for lua error handling and includes
- Added mg.onerror, mg.include, mg.redirect
- mg.onerror, if overridden, lets the user trap syntax errors and
  other loadbuffer errors, and dump stack traces and debug info
  for uncaught runtime errors.
- Log unexpected errors from mg.onerror handlers
- Provide mg.cry so custom handlers can reproduce default behavior
- Call mg.onerror handler before unwinding stack
- If mg.onerror returns non-0, stop loading chunks
- Included pages stop parent page processing if mg.onerror returns non-zero
- Included pages don't sent HTTP errors, they throw lua errors
Added http_error() to struct mg_callbacks
Updated readme
Updated readme
updated readme
avoid defining macros already present on mingw32
squash mingw printf format warning
squash mingw variable set but not used warning
portable socket type should be SOCKET
Squashed warning