Use gson to tidy up code

[knjk04]

• Could be used when storing the retrieved data into a Book object
• Could be used to format the saved data nicer (in Preview.java) with pretty printing
  • If using gson, then can remove the BookshelfJSON class

[knjk04]

Using in commit https://github.com/knjk04/Bookshelf/commit/ff7786a55fc0eac599450d1bc215019067e2cf68