add flannel plugin

This introduces a notion of a "meta" plugin. A meta plugin
is one that delegates the actual work of setting up the interface
to the main plugin. The meta plugin is used to select and dynamically
configure the main plugin. The sequence of events, is as follows:

Given netconf like:

    {
	"name": "mynet",
	"type": "flannel",
	"delegate": {
	    "type": "bridge"
	}
    }

flannel fills in values like "mtu", "ipam.subnet" and delegates to
"bridge" main plugin. "bridge" plugin will operate as usual, calling
into ipam module for IP assignment.

Delegate dictionary should not contain "name" field as it will be
filled in by the flannel plugin.

README.md

@@ -17,6 +17,7 @@ Hence we are proposing this specification, along with an initial set of plugins
 
 ## Included Plugins
 This repository includes a number of common plugins that can be found in plugins/ directory.
+Please see Documentation/ folder for documentation about particular plugins.
 
 ## Running the plugins
 The scripts/ directory contains two scripts, priv-net-run.sh and docker-run.sh, that can be used to excercise the plugins.