Added support for lua error handling and includes
- Added mg.onerror, mg.include
- mg.onerror, if overridden, lets the user trap syntax errors and
  other loadbuffer errors, and dump stack traces and and debug info
  for uncaught runtime errors.
- Log unexpected errors from mg.onerror handlers
- Provide mg.cry so custom handles 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