Merge pull request #459 from andrejbauer/master

Improved solutions to exercses 1.1, 1.2, and 1.3, see #436
missing primes on torus induction added
Merge pull request #466 from benediktahrens/torus_fix

missing primes on torus induction added
errata for defeb8c
remove spurious merge marker in errata
add errata markers for a lot of recent merges
close #448
close #450
unmarked errata for 88ab6d2
unmarked errata for 7d525ce
close #463
unmarked errata for e51d87f
close #445
unmarked errata for f8dab6a
add a bit of detail to the proof of 4.7.1
correct the proof of 2.11.1
refer back to 2.11.1 from 2-out-of-6 exercise
unmarked errata for 5702513
fix to 1.4
close #464