Export Changeset class to allow overriding

[snewcomer]

This PR would allow users to import the Changeset and extend from it.

import { EmberChangeset } from 'ember-changeset

And use like so

changeset(model, validationFn, validationMap, { changeset: ModifiedChangesetKlass })

Also the recommended way to generate new changesets is through a factory function instead of a class and the README has been updated. I'm not sure why the original implementation had a class w/ constructor but the factory function seems better. No breaking changes though.

import { Changeset } from 'ember-changeset

close #174 #390

[snewcomer]

@josemarluedke fyi