add support for lua includes
Added mg.onerror, include renamed to 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
Better support for mg.onerror handler: call before unwinding stack
If mg.onerror returns non-0, processing will abort.
Better error support for mg.include:
- 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