Make DeckTask a bit more defensive.

Add an easy way to check things are called from the right task and add
that to DeckTask.

Add an easy way to log methods being called.
Merge pull request #30 from flerda/defensive

Make DeckTask a bit more defensive.
Reformat CardBrowser.java.

This is to actually help merge another file that has been reformatted,
but it is always good to improve our code.
Merge pull request #38 from flerda/v2.1-card-browser

Reformat CardBrowser.java.
Upgrade Gson library to 2.2.3.

This now includes the full distribution, including sources, javadoc, and
README and LICENSE files.
Merge pull request #39 from flerda/v2.1-gson

Upgrade Gson library to 2.2.3.
Preferences and strings for collection preferences
Correctly persist and retrieve collection preferences.