Those without a keybase user account should still be able to use the cli to encrypt messages for those who do

The creation of this issue was triggered by this conversation: https://twitter.com/bepsays/status/500330631931625472

Is there any reason why a non-keybase user couldn't grab the `keybase` cli and use it to encrypt a message for another user that does have a keybase account?

The error caused when trying this seems like it could be avoided, i.e. it isn't actually critical to for both users to have a keybase account, but the cli currently assumes it and fails if the encryptor doesn't.

Here's how to repro:

Assumping you already have the following installed:

```
$ keybase version
keybase (keybase.io CLI) v0.4.15
- node.js v0.10.28
- gpg (GnuPG/MacGPG2) 2.0.22
- libgcrypt 1.5.3
Identifies as: 'keybase.io node.js client v0.4.15 darwin'
```

1) Move your `.keybase*` directories out of the way:

```
mv ~/.keybase ~/.keybasebak
$ mv ~/.keybase-installer/ ~/.keybase-installerbak
```

2) Try to encrypt a message as usual:

```
$ keybase encrypt -m"Top secret" chris
```

3) Note the exception as follows:

```
warn: No config file found; tried '/Users/cbeams/.keybase/config.json'

/usr/local/lib/node_modules/keybase/lib/env.js:396
return (u2 != null) && (u2.toLowerCase() === this.get_username().toLowerCas
^
TypeError: Cannot call method 'toLowerCase' of null
at Env.is_me (/usr/local/lib/node_modules/keybase/lib/env.js:396:72)
at iced.Deferrals.parent (/usr/local/lib/node_modules/keybase/lib/command/encrypt.js:166:21)
at Command.exports.Command.Command.run (/usr/local/lib/node_modules/keybase/lib/command/encrypt.js:210:15)
at /usr/local/lib/node_modules/keybase/lib/command/all.js:271:23
at /usr/local/lib/node_modules/keybase/lib/command/all.js:280:13
at Deferrals.exports.Deferrals.Deferrals._call (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:86:16)
at /usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:98:26
at exports.trampoline.trampoline (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:64:14)
at Deferrals.exports.Deferrals.Deferrals._fulfill (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:96:16)
at ret (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:29:18)
```

Expected behavior would be that the cli doesn't require the encryptor to have a keybase account, and just proceeds as per usual.

Reproduce the failure with the documented keybase encrypt command after moving the .keybase directories. Start in lib/env.js at Env.is_me and inspect its call from lib/command/encrypt.js. Done means a user without a Keybase account can encrypt a message for an existing user without the null-method exception.

Written by the indexing model from the issue text.