Assertions for SharedPreferences added.

[aahlenst]

Would you prefer a message like it's formatted in fest-assert?

java.lang.AssertionError: expecting:
<{'SOME_KEY'='[]', 'ANOTHER_KEY'='["bar","foo"]'}>
 to contain:
<[MapEntry[key='SOME_KEY', value='["foo","bar"]']]>
 but could not find:
<[MapEntry[key='SOME_KEY', value='["foo","bar"]']]>

Or do you generally dislike printing collections in their entirety? I'm just asking so I know for the future.

[JakeWharton]

@aahlenst I'd like to switch to using proper formatted message in the future but I want to switch the whole library at once.

For now I think omitting the entire map and just saying "preferences" is fine.

[aahlenst]

@JakeWharton Would you like to see further changes before merging?