Allow valid ssh public keys with options.

SSH Public keys can include options before the ssh type. This PR changes the way the public key is parsed to allow those options.

For example:

```
from="*.eng.cam.ac.uk",no-X11-forwarding,noagent-forwarding ssh-rsa 
AAAAC4MybC1yc2EAAAABIwAAAQEAybmcqaU/Xos/GhYCzkV+kDsK8+A5OjaK5WgLMqm
u38aPo56Od10RQ3EiB42DjRVY8trXS1NH4jbURQPERr2LHCCYq6tHJYfJNhUX/COwHs
+ozNPE83CYDhK4AhabahnltFE5ZbefwXW4FoKOO+n8AdDfSXOazpPas8jXi5bENf7he
ZT++a/Qxbu9JHF1huThuDuxOtIWl07G+tKqzggFVknM5CoJCFxaik91lNGgu2OTKfY9
4c/ieETOXE5L+fVrbtOh7DTFMjIYAWNxy4tlMR/59UVw5dapAxH9J2lZglkj0w0LwFI
+7hZu9XvNfMKMKg+ERAz9XHYH3608RL1RQ== This comment describes key #2
```

See http://www.eng.cam.ac.uk/help/jpmg/ssh/authorized_keys_howto.html