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
Populating fields based on spreadsheet selection data

- Revised gulp.js to ensure service is added last in "angular" task
- Directive now manages and watches new scope values for binding and scope spreadsheet value updates
- Template updated to apply the anchor tag for displaying the Spreadsheet name as a hyperlink. Also now uses ng-options for binding select choices and a new scope value as its hg-model
- Incorrect injection array removed in service file and also "filterSheets" function fixed to properly use iteration values in angular.forEach
- e2e test HTML file updated to use service when debugging locally
Merge pull request #17 from stulees/chore/spreadsheet-data

Populating fields based on spreadsheet selection data