Create ConversionService

Adds a new HandlerService: ConversionService, operating on
ConversionHandler plugins.

The purpose of these handlers is to process a new class,
"ConversionRequest", which signifies the desire to cast or convert an
object of one type to another.

This will take the place of ConversionUtils, as it is an extensible
framework. ConversionHandler plugins can decide what types of conversion
they support and thus override the default behavior (the default plugin
supports primitive : object conversions and cases where the destination
object has a constructor that will accept the source, for example).

Closes https://github.com/scijava/scijava-common/issues/95
ConversionUtils: delegate to ConversionService

The ConversionUtils canConvert and convert methods will now all delegate
to a ConversionService, if available, or a DefaultConversionHandler if
no ConversionService has been set.

The ConversionUtils class will be retained, as it provides some API
which does not need generalization via a Service/plugin framework. All
delegating methods have been marked deprecated however, and consumers
should switch to ConversionService usage if possible.

Updated the AbstractConversionService so that ConversionServices set
themselves as the delegate service in CovnersionUtils on initialization
(only the highest priority will "stick").