Re-organize class and package names around pluralization standards

* https://softwareengineering.stackexchange.com/a/75929
* http://english.stackexchange.com/q/25713

Some thoughts:

* Singularize all classes
* Don&rsquo;t hew so closely to E\*TRADE API conventions (since they clearly don&rsquo;t have any)
  * `orderPreview` -> `previewType` or `orderType`
