AddressBook.java: add recover command

[CaesarTY]

seedu/addressbook/AddressBook.java: add recover command
test/expected.txt: modified the expected information
docs/UserGuide.adoc: add recover information

[chemry]

Bugs need to be fixed:

1. case that no delete operation before recover
2. case that input two consecutive recover command

[CaesarTY]

@Happytreat

[Happytreat]

Good job adding a new recover command and following the coding standards :)

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