Fix various typos

Squashed commit of the following:

commit 040fd169bfd6845b33d3f86fd66afb4a632605c6
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Nov 30 22:24:30 2014 +0000

    Fix more typos in comments

commit 6121ae593008a574735427e047fdb7a16f4fa47f
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Nov 30 22:20:37 2014 +0000

    Fix more typos

    Not all are in comments this time, but the vast majority are, and
    the rest are in printed text.  None are variable or constant names.

commit 84bc72573009bb46f5601f7257a7f7538f25cfc2
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Nov 30 22:01:46 2014 +0000

    Fix some typos
Merge pull request #59 from ZoeB/master

Fix various typos