ECDSA signing of app name as the addon URL password

Adds functions for signing a string with [Elliptic Curve DSA](https://en.wikipedia.org/wiki/Elliptic_Curve_DSA).

I've added app name signing to the API. It now includes the signed app name as the password in `WEBX_URL`.

I also added a CLI tool, `webx-keygen`, which generates and base64-encodes a public & private key set:

```
$ ./webx-keygen 
Generating 570 keys...
Private key D:	 34894740695352199621132960150595114337912715064695366406784853155838494334525
Public key X:	 8346418768908771457504298428640031862436379533854562700975454173970101264546
Public key Y:	 78863740101368281075532775253290569269590858638019266577433221724057309031455 

PrivateKey base64:
TSW5LCNwLcVkP17t7yDW0RgrvrPXi0np1z7yyr2BXj0= 

PublicKey base64:
EnPn1langquwCLaewAmbo0PlrRXWgE9clI4ft695YKKuW0uKgIlHwHwMx7klLr8_g1R0i8-07BLKZmnR4tQsHw==
```

Haven't integrated it into webxd and webx-router yet. I'm assuming you'd prefer that the password just gets added as another field in the `BackendCommand` struct such as `Auth` or `Password`?

Also I could probably use at least a quick review w/ @tmaher