GetTimeEntries: given default values for start_date and end_date break the call
GetTimeEntries: added `at` to service parameters

@see https://github.com/toggl/toggl_api_docs/issues/114
Revert "GetTimeEntries: added `at` to service parameters"

This reverts commit f503abecf52a27fd9efb8e0f95fce6577084e51c.
Merge pull request #3 from derhasi/time_entries

Update service definition for getTimeEntries