Improve Crypto::Box documentation

Slightly improve the text in the opening paragraph, as well as the short
summary of the class.  Also add a ficticious exchange between our old
friends, Alice and Bob and their somewhat vindictive foe, Eve.
Rename authenticator specs in prep for one time

This allows for greater consistency between all the authenticators!
One time authenticators

API for making one time authenticators.  Currently not that useful, at
least without an easy way of generating keys which can be used by both
sides of a conversation.  But anyway, its here.
Merge pull request #11 from namelessjon/dev/one-time

One time authenticators
Merge pull request #9 from namelessjon/docs

Improving the docs
Process docs with markdown
Use message, not data for arguments

This is more consistent with the specs, and also with the NaCl
documentation, which is now linked.