Add HTTP User-Agent to fetchJSON

[sadlassian]

This adds an HTTP User-Agent to all of the requests made by the client.

Recently, there was an issue where Trello put api.trello.com behind CloudFront, which caused GET requests with an empty body to return a 403 status code (see: https://github.com/sarumont/py-trello/issues/373).

A fix for fixing GET requests with a body is already pending (see: https://github.com/sarumont/py-trello/pull/374).

For future possible issues, adding a User-Agent to this library will help Trello SREs identify where affected traffic is coming from, which means a better experience for users!

[veracus]

When can we expect this to be merged? Are you looking for more maintainers? cc: @sarumont

[kdinn]

Thanks for working out a fix to this issue folks. My site is not talking to Trello any more because of it.

I second the question above: when can we expect this to be merged?

[Kab00mKap0w]

See discussion re: repo's future