Require a custom user agent to use the SDK

[teolemon]

What

• Require a custom user agent to use the SDK
• Using the SDK and the API in general will remain as of today: free and accessible for everyone.
  IN the same way, we don't want to implement an API key mechanism and all that kind of stuff.
• However, due to misuse of the API through various SDKs, we will now force all language clients to have a valid User Agent so that we can reach out in case of problems.
• For now, We will only require an app name for the user agent.

[nicolasleger]

@teolemon which value for this gem?