Feature: add support for server-side rendering

[DethAriel]

As of the moment, including re-captcha into a server-side rendered page will result in an error (see #33). The component should gracefully handle server-side rendering instead of erring out.

[mohitkadelongraph]

Angular version : 4
We we can fix it by wrapping window and document in isPlatfromBrowser.
RecaptchaLoaderService class in recaptcha-loader.service.ts

import { ... , PLATFORM_ID, Inject } from '@angular/core';

import { isPlatformBrowser, isPlatformServer } from '@angular/common';

// Injecting platform id in constructor
constructor(@Inject(PLATFORM_ID) private readonly platformId: any, ...) {
    ...
    if(isPlatformBrowser(this.platformId)) {
      this.ready = RecaptchaLoaderService.ready.asObservable();
    }
    else {
      this.ready = Observable.of();
    }
}

private init() {
    if (RecaptchaLoaderService.ready) {
      return;
    }
    // isPlatformBrowser condtion
    if(isPlatformBrowser(this.platformId)) {
        window['ng2recaptchaloaded'] = () => {
          RecaptchaLoaderService.ready.next(grecaptcha);
        };
        RecaptchaLoaderService.ready = new BehaviorSubject<ReCaptchaV2.ReCaptcha>(null);
        const script = document.createElement('script') as HTMLScriptElement;
        script.innerHTML = '';
        const langParam = this.language ? '&hl=' + this.language : '';
        script.src = `https://www.google.com/recaptcha/api.js?render=explicit&onload=ng2recaptchaloaded${langParam}`;
        script.async = true;
        script.defer = true;
        document.head.appendChild(script);
    }
}