Allow `Highrise::Subject` finder methods to accept an optional hash of options

Allow `Highrise::Subject` finder methods to accept an optional hash of options which are then passed to the appropriate find method. This lets us pass `since` when fetching, for example, a Person's Notes, or a Comapny's emails.

Before:

`person.emails # Have to get all of them`

Now:

`person.emails(:params=>{:since=>"201408051230"})`