added edit view, updated add/library view, updated add/edit[book] templates

changes in views.py:
- library view:
- - - returns author and category names, insted of id's

- add_book view:
- - - additionally returns book authors
- - - minor fixes

- edit_book view:
- - - change category, by selecting from available categories
- - - change author, by selecting author name, it will be transfromed
to id properly
- - - change title
- - - change amount

changes in templates:
- added edit book
- improved add book and library (adds meaningful display to disabled
books)