Generate shorter tokens

Following on from @paulfurley's great change to /dev/urandom in
b4eafed, I've implemented @robyoung's
comment on #262 to shorten the token length to 64 characters.

I've also increased the characters included from a-f0-9 to a-zA-Z0-9.

I don't really want to make big assertions about cryptography, but
this has reduced the key space from 10^154 to 10^114, so I think
we're probably still ok.

tools/generate-token.sh

@@ -1,7 +1,12 @@
 #!/bin/bash
 
 #
-# Generate a random token 
+# Generate a random token
 #
 
-head -c 500 /dev/urandom | shasum -a 512
+# LC_CTYPE=C is specified for OS X, as otherwise tr will return
+# an illegal byte sequence from assuming /dev/urandom is UTF-8
+
+cat /dev/urandom | LC_CTYPE=C tr -cd 'a-zA-Z0-9' | head -c 64
+
+echo '' # Add a newline