Find method

[DivineOmega]

This PR provides the ability to find Xero models by their GUIDs, similar to the syntax of the Eloquent find method.

# Retrieve an individual contact by its GUID
$contact = $xero->contacts()->find('34xxxx6e-7xx5-2xx4-bxx5-6123xxxxea49');

# Retrieve multiple contact by their GUIDS
$contacts = $xero->contacts()->find([
    '34xxxx6e-7xx5-2xx4-bxx5-6123xxxxea49',
    '364xxxx7f-2xx3-7xx3-gxx7-6726xxxxhe76',
    ]);

[jaredkove]

Nice addition.