Adding CDA identifiers to providers

Making CDAIdentifier embedding polymorphic. This ripples through to
entry. Providers can now have many CDAIdentifiers. This is to support
identifiers for providers in QRDA Cat I.
Refactoring the way that NPI and TIN are stored in provider

Previously, these were String fields on Provider. Now they are
stored in CDAIdentifers. Convenience methods have been created to
allow for them to be acccess the way they used to be.
Merge pull request #107 from projectcypress/identifiers_for_providers

Adding CDA identifiers to providers