`Strategy` constructor.

The Bitbucket authentication strategy authenticates requests by delegating to
Bitbucket using the OAuth 2.0 protocol.

Applications must supply a `verify` callback which accepts an `accessToken`,
`refreshToken` and service-specific `profile`, and then calls the `done`
callback supplying a `user`, which should be set to `false` if the
credentials are not valid.  If an exception occured, `err` should be set.

Options:
- `clientID`      identifies client to Bitbucket
- `clientSecret`   secret used to esablish ownership of the client key
- `callbackURL`      URL to which Bitbucket will redirect the user after granting authorization
- `scope`         array of permission scopes to request.  valid scopes include:
(see https://confluence.atlassian.com/display/BITBUCKET/OAuth+on+Bitbucket#OAuthonBitbucket-Scopes for more info)

Examples:

passport.use(new BitbucketStrategy({
clientID: '123-456-789',
clientSecret: 'shhh-its-a-secret'
callbackURL: 'https://www.example.net/auth/bitbucket/callback'
},
function(accessToken, refreshToken, profile, done) {
User.findOrCreate(..., function (err, user) {
done(err, user);
});
}
));

@param {Object} options
@param {Function} verify
@api public
function Strategy(options, verify) {
  options = options || {};
  options.authorizationURL = options.authorizationURL || 'https://bitbucket.org/site/oauth2/authorize';
  options.tokenURL = options.tokenURL || 'https://bitbucket.org/site/oauth2/access_token';
  options.customHeaders = options.customHeaders || {};

  if (!options.customHeaders['User-Agent']) {
    options.customHeaders['User-Agent'] = options.userAgent || 'passport-bitbucket';
    //HACK: requests need to fall back to Basic Auth (for access_token call)
    options.customHeaders.Authorization = 'Basic ' + new Buffer(options.clientID + ':' + options.clientSecret).toString('base64');
  }

  OAuth2Strategy.call(this, options, verify);
  this.name = 'bitbucket';
  this._userProfileURL = options.userProfileURL || 'https://api.bitbucket.org/2.0/user';
  this._oauth2.useAuthorizationHeaderforGET(true);
}