Run promise in isReady instead of constructor

[mborne]

It will avoid unhandledRejection from the constructor

(refs #138)

[mborne]

Tell me if you are interrested in some hardening like the following in this PR or another one :

• "ensure that isReady() is called" to avoid misuse of the class.
• param initialization (i'm leaving the warning Property '_info' has no initializer and is not definitely assigned in the constructor. to avoid noise)
• type completion and comment fixing

Something like :

class WfsEndpoint {
  /**
   * A Promise which resolves to the endpoint information.
   */
  getServiceInfo() {
    return this._info;
  }
}

=>

class WfsEndpoint {
  
  // ...

  ensureIsReady(){
     if ( ! this._info ){
         throw new EndError("isReady() must be resolved before calling WfdEndpoint methods")
     }
  }

  /**
   * Get the endpoint information.
   */
  getServiceInfo(): GenericEndpointInfo {
    this.ensureIsReady();
    return this._info;
  }

}