* Replace DNSimple API client with the official Go client
* Upgrade DNSimple provider to use the new API v2
Acceptance tests pass:
```
=== RUN TestProvider
--- PASS: TestProvider (0.00s)
=== RUN TestProvider_impl
--- PASS: TestProvider_impl (0.00s)
=== RUN TestAccDNSimpleRecord_Basic
--- PASS: TestAccDNSimpleRecord_Basic (2.67s)
=== RUN TestAccDNSimpleRecord_Updated
--- PASS: TestAccDNSimpleRecord_Updated (1.88s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/dnsimple
```
Note that the code still has to be updated to pass the account ID
dynamically in place of "TODO-ACCOUNT".
* Refactor DNSimple provider to expose both client and config
The config is required as the new API wants to know the identifier of
the account you are operating to. The account is not stored in the
client (as the client can talk with different accounts), hence I need
to pass it as part of the config.
* Identify Terraform requests to DNSimple via UserAgent
* Upgrade to the latest dnsimple-go version
* Update docs
Provide upgrade instructions and update the docs for API v2.
* Remove rendundant type declaration
