Enforce valid choices

When a choice is given, the user must enter a valid option to continue.
Fixed spelling mistake

'aan' changed to 'an'
Update object-orientated-library.py
Update object-orientated-library.py
Merge pull request #1 from liam-m/patch-1

Enforce valid choices
Cleanup, changed book to subclass of item
Fixed bug where item.setUnavailable() wouldn't work

Changed self._unavailable = False to self._available = False
Update object-orientated-library.py

Changed class names to CapWords in accordance with PEP 8
Added members list to Library class
Cleanup

- Changed input and raw_input text
- More PEP 8 compliance
Fixed inheritance for Book

Book now calls Item's init function
Fixes

- Changed Library class to store items and members as dictionary
- Fixed adding members to Library
- Used int(raw_input()) to get integers rather than input()
- Fixed check for uniId in Library items
- Small grammar fixes in input text
- Fixed itemFunctions to work correctly rather than copy and pasted
memberFunctions
Added in Item getters
Fix for getUniId
* prompt_input() function to reduce code repetition
* initialisation function for DVD, CD, Game
* getters and setters for DVD and Game certs
* members current items is now [] rather than None
* member and item IDs are generated by the library class to prevent
clashes
* handle member/item not found in viewMember() and viewItem()
* started to make rentItem() and returnItem() in Library class and
checkInOut() function
Small change to prompt_input
Merge pull request #3 from liam-m/master

Fixes many bugs I couldn't be bothered with at the time.