Add debugger to development dependencies

* debugger if ruby > 1.9
* else ruby-debug
Permit passing of field_list to finder methods.

This is to address issue we are facing where sobjects created in
salesforce have large numbers of fields (in some cases ~ 470), which are
unable to be queried for as SalesForce returns an error indicating the
generated query is too long
Add some field_list options to README
Try and make specs tolerant of ruby 1.8 where hash order is different
Remove debugger gems again - probably not necessary permanently.
Merge pull request #2 from smulube/field_list

Optional field list restrictions for queries