added support for server side rendering (angular universal)

[samuel-p]

I need support for server side rendering using angular universal. The problem with this is that there is no document on server side, so you have to check if the dom exists before using it:

if (isPlatformBrowser(platformId)) {
    // ...
}

I added that check in your DocumentService to be able to use this lib with angular universal.

[tnicola]

Thanks for this PR @samuel-p !