Email address is optional in uids

GPG lets you make uids without an email address, and some people use this feature to do stupid crap :).

```
pub   8192R/B1D88291 2014-02-15 [expires: 2021-02-13]
uid                  Sidney San Martín (Born 1989-7-1 in San Francisco, CA)
uid                  Sidney San Martín <sidney@s4y.us>
uid                  Sidney San Martín <sidney@okcupid.com>
sub   8192R/59FEA86F 2014-02-15 [expires: 2021-02-13]
```

`parse` was returning `null` for one of my uids, which keybase/node-client didn't like. So, I changed it to make email address optional, and made the capture groups a little simpler (I hope!).