Move signature generation at the bottom, feels more cohesive
Improve readability, indenting and fixes (see description)

- `include_once('OAuth.php');` instead of `include_once('oauth.php');`
- `$this->signature_method` to `$signature_method` since we don't have
the signature method defined at this point to we add this
`$signature_method = new OAuthSignatureMethod_HMAC_SHA1();` which
wasn't included