Add slashes for RFC6920 ni:// URIs

[RFC6920](http://tools.ietf.org/html/rfc6920) ("Naming Things with Hashes") defines two new URI Schemes: "ni" and "nih" for use with Named Information.
The first one (ni) needs the double-slashes added. The second one (nih) is a human readable version and does not.

Examples:

	ni:///sha-256;UyaQV-Ev4rdLoHyJJWCi11OHfrYv9E1aGQAlMO2X_-Q
	nih:sha-256-120;5326-9057-e12f-e2b7-4ba0-7c89-2560-a2;f

This patch adds the ni-scheme to the list of protocols that get slashes added and also notes this in the documentation.