Spec out the default ping callback and remove wm_reqdata include
Make wm_reqdata opaque, creates a bunch of warnings.
Update tools.mk
Reorganize exports
Cleanup wrq:create() and record initialization, types

* Removed meaningless field values that say where the field is populated
* Put defaults in the record definition where possible
* create/1 was poorly named, so renamed it to parse_uri_and_cookies/1
* Exported response_body/0 type
* Renamed req_headers/0 type to headers/0 since it is used for resp too