AWS client tools commonly support passing credentials via
`AWS_ACCESS_KEY_ID` + `AWS_SECRET_ACCESS_KEY`, but supporting only this is
insufficient. For example, access key IDs provided by STS require passing in
`AWS_SECURITY_TOKEN` as a third value, and EC2 instances are often provided
dynamic credentials at runtime via the EC2 metadata service.
This changeset makes `lego` attempt to find credentials in the same way that
the `aws` CLI tool attempts to find credentials. The result is even less
auth code than before because `goamz` provides all this with `aws.GetAuth()`.
- InvalidTXTRDATA error when creating TXT record (closes#94)
- Present() should poll and wait until the status of the record change becomes INSYNC (thanks @oov)
Adds a retry/timeout utility function to dns_challenge.go that may be used in other places
* new ChallengeProvider with Present and CleanUp methods
* new Challenge type describing `http-01`, `tls-sni-01`, `dns-01`
* new client.SetChallengeProvider to support custom implementations
