Version 2.3 of the SOAP API can be safely used

The difference between version [2.2][2.2] and [2.3][2.3] of the WSDL contract<sup>†</sup> is subtle enough not to affect the current codebase.

<sup>†</sup>_addition of `activityNameFilter` to the `GetLeadChanges` params and the removal of `skipActivityLog` from the `ListOperation` params_

  [2.2]: http://app.marketo.com/soap/mktows/2_2?WSDL
  [2.3]: http://app.marketo.com/soap/mktows/2_3?WSDL
Merge pull request #8 from mariusbutuc/patch-1

Version 2.3 of the SOAP API can be safely used
The API doesn't let you create custom attributes

Document that the Marketo API does not let you create custom fields at this time – follow-up on the discussion in PR #1.
Merge pull request #9 from mariusbutuc/patch-2

The API doesn't let you create custom attributes