Write Unit Tests for the `persistence` module

[scottrice]

Things to test:

[BackedObject]

• find returns None with an invalid identifier
• find returns the same object when called twice with the same identifier
• all returns an empty list when there are no identifiers
• backed_value returns the correct value

[ConfigFileBackingStore]

• identifiers returns the correct value
• An identifier added with add_identifier shows up in subsequent calls to identifiers
• An identifier removed with remove_identifier doesn't appear in a subsequent call to identifiers
• add_identifier raises ValueError when an identifier already exists
• remove_identifier raises ValueError when that identifier doesn't exist
• keys raises ValueError when the identifier doesn't exist
• get returns the default when the key doesn't exist
• get returns the default when the value is empty string
• get is case insensitive with keys
• set is case insensitive with keys
• A value modified with set is different in subsequent calls to get
• save raises IOError when writing to the location is not allowed
• save raises IOError when the location doesn't exist

[scottrice]

Fixed by 24b1f2876b1a00be08c0ef5fdc16fe6038430f19