Add ability to mock responses

[mdb1]

Add ability to use mocked responses in the library instead of actually hitting the network.

This will be useful for quickly testing behaviors (success/error responses) and for the unit tests to avoid hitting the network.

Ideally, we might want to have:

• Ability to set global approach (mock vs real)
• Ability to add delay for the mocks (to be able to visually check the loading states)
• Ability to use a mock response for a specific request (vs global approach)
• Ability to load mock responses from json files
• Ability to load mock responses from in-lined json strings

I think @juansanzone has some great ideas on how to accomplish this. Feel free to raise any input/concerns you may have 😄