Prevent creation of duplicated names

[lavigne958]

current behavior

When creating a spreadsheet, Google API does not block when an other Spreadsheet exists with the same name. It will create a second one with the same name.

Possible improvement

Add an option on method create that will first check if a spreadsheet with the name exists before creating a new one.

This could prevent creating a spreadsheet with a name, loosing it's ID then trying to create it again and create a duplicate of the first one.

errors cases

If the spreadsheet already exists 2 possibles solutions to warn the user about it:

• raise an exception (SpreadsheetExists) and do not create a new one. This will maintain a unique path for each action, create only create a new non existing file and open only opens existing files.
• open and return the spreadsheet with no error.
  This will change the behavior of gspread but can be handy for a lot of users.

@aiguofer @burnash
What do you think is best?

[lavigne958]

Relates to #359

[lavigne958]

Won't do, we should not prevent users from creating spreadsheets if the API allows it.