Fix URL to http://homotopytypetheory.org

The current URL http://www.homotopytypetheory.org is a
permanent redirect (301) to http://homotopytypetheory.org,
so the latter URL should be used instead.
Fix typo in exercise 1.4 from "C_0" to "c_0"

See also: http://homotopytypetheory.org/#comment-4259
Merge pull request #342 from vog/fix-c0

Fix typo in exercise 1.4 from "C_0" to "c_0"
Add missing "(x)" to "prd(x:A) B"
Merge pull request #345 from vog/fix-bx

Add missing "(x)" to "prd(x:A) B"
Fix #343
Merge pull request #341 from vog/fix-url

Fix URL to http://homotopytypetheory.org (this is also what's used at other places in the book)
bring errata up to date
Don't use traditional propositional notation before it's introduced, fix #346