adding pluggable module support to recon middleware

allows a swift administrator to extend recon metering with discoverable modules.
entrypoints added to system in name space "swiftrecon.plugin" are added under
the /recon/plugins/[plugin-name] path.

example plugins can be found at https://github.com/ahale/recontools which provides
some trivial examples.

an example of usage could be to provide information about underlying disk controllers
or arrays. enabling richer information through recon without having to add to
swift itself.