Enable Custom Password Checking

Configuration can now specify a function to validate the password during authentication:

```js
var crypto = require('crypto');

// check the hash
function hash(raw, encoding){
  return function(password){
    var h = crypto.createHash('sha256');
    h.update(password);
    return h.digest(encoding || 'hex') === raw;
  };
}

exports.config = {
  users: {
    admin: {
      password: hash('9c0d9ae0fcf7572ca9a48eaa8b4d40730dc0f4e24ace3b38c35d34cb9fd1665e'),
      cwd: '/',
    }
  },
  port: 8080
};
```