Add sqerl_rec:qfetch/3 fetch by named query returning record


    This new function provides a convenient interface for custom defined
    prepared queries that return records with a corresponding module
    implementing the sqerl_rec behavior.

    The qfetch function also cleans up some repitition among the fetch
    functions.

    Also added ensure_error to cleanup error returns and make sure they
    are always of the form {error, Why}.

Also adds docs for sqerl_rec and checks generated docs into doc/ for github viewing.

ping @christophermaier @jamesc