`gravatar`- Add API Key initialization to the SDK

[hamorillo]

Closes #156

Description

Adding the method to initialize the SDK with the API Key. At the moment, that API Key is required by the new REST API when you want to get a full profile. Without the API Key, some fields are not returned by the backend because you need to be authenticated.

Also adding in the DemoApp an expandable section that allows to review the profile raw data:

Testing Steps

• Add your ApiKey in local.properties as follows:

demo-app.gravatar.api.key=YOUR_API_KEY_HERE

• Launch the DemoApp

• Go to the Profile tab

• Get any profile

• Expand the Raw Profile

• Verify it contains fields only present within authenticated requests like: registrationDate, lastProfileEdit, contactInfo etc.

• You can also verify that you don't receive those fields when the ApiKey is removed from local.properties.