explain crypto
and encapsulation
simplify encapsulation diagrams
by dropping the standard packet headers
Merge branch 'master' into 40_crypto_docs

Conflicts:
	site/how-it-works.md
some rewording and some links
fix typo
make it clear that key pairs are created per connection
cosmetic
state that nonce prepending is normal in NaCl
fix typo
Typos
Merge PR #269 from 'rade/40_crypto_docs'

Addresses #40