Fix dd blocksize parameter

When specifying a blocksize in megabytes you need to use a capital M. Otherwise you get this error:

```
wichert@kim $ dd bs=1m if=2014-01-07-wheezy-raspbian.img of=/dev/disk1
dd: invalid number '1m'
```
Reword to document both osx and GNU dd options

This is needed because installing GNU coreutils can add GNU's dd
to $PATH, which takes a different syntax.
Merge pull request #44 from wichert/patch-1

Fix dd blocksize parameter