Store authentication information in keychain, rather than user defaults

This change brings in UICKeyChainStore for easy use of the keychain.
Clear out old keychain tokens

When an application is uninstalled, the keychain values will persist.  In order to prevent
using any old keychain tokens, we clear them on first launch of the application.
Remove apiToken method from Coinbase

Removed apiToken method from Coinbase, as the CBTokens class can be used instead.
Merge branch 'keychain'
Updated to use new coinbase API endpoint

The old API endpoint had been deprecated and this change updates to use the new API endpoint.