Adding service for retrieving worksheets from spreadsheet data

- One function "getSheets" which requires a fileId (obtained from outside use of Google Drive Picker)
- "getSheets" chains promises starting with getting the JSON data, then configuring the data to provide an array of worksheets, and finally catching an error
- Defined constants to help in decomposing
Merge pull request #16 from stulees/chore/spreadsheet-data

Adding service for retrieving worksheets from spreadsheet data