Add two methods including VerifyCredentials. Add Rate Limit checking capability.

Two methods were added to `TwitterApi`: `GetSelf()`  and `VerifyCredentials()` which both request at `/account/verify_credentials`

Errors thrown by `decodeResponse` are of a new type: `*ApiError` which returns the same string as legacy to `err.Error()`. But, this new error type can now be used to determine Rate Limit errors:

```go
if apiError, ok := err.(*ApiError); ok {
  if rateLimited, nextWindow := apiError.RateLimitCheck(); rateLimited {
    time.Sleep(nextWindow.Sub(time.Now()))
  } else {
    panic(err)
  }
} else {
  panic(err)
}
```