Guard MdIconModule if not called .forRoot()

[mhevery]

Currently it is easy to forget to call MdIconModule.forRoot() Could we throw an error like we do in Router: https://github.com/angular/angular/blob/master/modules/@angular/router/src/router_module.ts#L123

[devversion]

This should be fixed with SHA b49bfce

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