Added Random Token Generator.

Added a random token generator that generates a random string from digits, uppercase and lowercase characters, the function can be used as: 
```
var Faker = require('Faker');

var length = 10;
var randomToken = Faker.Token.generateToken(length);
```
This could be used to generate fake password data, or generating tokens in 'forgot my password' reset emails. I'd write tests for this if it wasn't random :)

Cheers !