fix(icon): export icon registry as part of icon

[kara]

Currently, if you want to use md-icon, you have to import MdIcon and MdIconRegistry from separate files. This PR consolidates the import, so you can do:

import {MdIcon, MdIconRegistry} from '@angular2-material/icon'

instead of:

import {MdIcon} from '@angular2-material/icon'
import {MdIconRegistry} from '@angular2-material/icon/icon-registry'

[jelbourn]

Now needs rebasing on my mega change #384

[jelbourn]

LGTM

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