Default DKIM keys with 1024-bit instead 512-bit

Seems like Google is not very happy with the 512-bit. There are many news and blogs posts about it. 
Example: http://blog.wordtothewise.com/2012/11/gmail-sending-out-warnings-for-512-bit-dkim-keys/

I personally tested it with 1024-bit and 2048-bit and everything is working just fine.
Merge pull request #124 from demlasjr/patch-1

Default DKIM keys with 1024-bit instead 512-bit