refactor: clarify the options keys of Gateway#payment_redirection

Gateway#payment_redirection expects an option named :base_uri instead
of :host. Sending :host is supported, but will print a deprecation
message

Closes #18
feat: Allow to configure a base_uri directly in omnipay
Fix url params in request phase
Add BitPay adapter
Add bitpay to the sample_app
WIP - omnipay 2.0 adapters refactoring