Refactor Prefs to PrefService

Using a static utility method for Prefs is limiting - it provides no way
to override preference saving/loading behavior. This creates problems
when there are errors or negative interactions with the Preferences
implementation - for example, in MATLAB there are known bugs regarding
the Java Preferences implementation.

By converting to a PrefService we can now control this behavior more
precisely, as needed. The Prefs utility class will remain for now, but
will delegate to the PrefService when it has been created - allowing the
utility method behavior to be overridden similarly.
Update Prefs usage to PrefService

All Contextual classes that used the Prefs static utility class should
now be using PrefService.
ModuleService: add save/load item methods

Added API to save/load ModuleItems. This will allow Contextual classes
to move away from ModuleItem#loadValue and ModuleItem#saveValue, which
are not Contextual and go directly through the Prefs utility class.

Adds a default implementation in DefaultModuleService, using the
PrefService if able (which requires an AbstractModuleItem). If
necessary, still falls back to ModuleItem's saveValue and loadValue
methods.
Mark saveValue and loadValue deprecated

ModuleItem#saveValue and ModuleItem#loadValue are now deprecated in
favor of ModuleService methods, which can be called from a Contextual
environment and thus use the new PrefService API.
Refactor for ModuleService saving and loading

Updated classes that were using ModuleItem#saveValue and
ModuleItem#loadValue to use the appropriate ModuleService methods.
AbstractPrefService: account for priority

It was discovered that the initialize method of multiple PrefService
impls was being invoked - leading to the LOWEST priority overwriting the
static service field in the Prefs utility class (since it was the last
initialize method to execute).

Added a check for priority so that lower priority services don't
overwrite higher.
Clean up code

Appled Source > Clean Up in Eclipse, to update newest classes with the
IJ2 style template.
DefaultPrefService: add TODO

Added comments detailing planned future for the pref service.
Deprecate Prefs

The Prefs utility class is now deprecated. PrefService should be used
instead.
PrefService: remove setStaticBehavior method

This was an unnecessary method. Originally thought that each service
would need a reminder to set itself as the static PrefService... but
that's exactly what the initialize method of AbstractPrefService is for.
PrefService: remove Preferences imports

The whole point of the PrefService is to allow for any preference
implementation, while the DefaultPrefService uses java.util.Preferences.

Thus the java.util.Preferences imports have been removed from the
PrefService, and the necessary Class and String-based indexing methods
have been added.

Updated the DefaultPrefService implementation and usage of the service
accordingly.
Rename preferences to prefs package

We have a short name PrefService, so we should use a short package name
instead of a long package name.
DefaultPrefService: fix javadoc
DefaultPrefService: do not reference Prefs class

The Prefs class is deprecated and we should not reference it in any way
from non-deprecated code.
Add down-the-middle unit tests for PrefService
AbstractPrefService: suppress warnings

Added suppress warnings for the Prefs deprecation.
Merge pull request #102 from scijava/create_pref_service

Migrates the static utility `Prefs` class to an extensible `PrefService`.