core: support rados_stat

Added stat support for lua-rados. Also updated test.lua
to reflect the new functionality.

Signed-off-by: Adam Manzanares <nmtadam@gmail.com>
Reviewed-by: Noah Watkins <noahwatkins@gmail.com>
core: fix whitespace

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
core: fix LDoc markup tag

@return rather than @returns

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>