#136
When the URL is found to be null and classLoaders is not null, assuming an extension is at cause, null the classLoaders and try again.  This should result in trying the DataCleaner default provided images.  If the classLoaders were specific to DataCleaner, then this will cause one more attempt.  However, I expect this to be a rare case.
#136
Fixed "input = null" dialog message.  This was due to missing return statement when trying to check the default class for images.
Merge pull request #137 from mhorner7wd/master

#136 Fixed exception when image not found in imported extension
Updated AB dependency to 1.0.4
Fixed deprecated .getInstance() calls
[maven-release-plugin] prepare release DataCleaner-3.7.2
[maven-release-plugin] prepare for next development iteration
Upgraded Guava dependency
Added CONTRIBUTE.md file for potential contributors
Merge branch 'master' into feature/29-graph-ui

Conflicts:
	core/pom.xml
	desktop/pom.xml
	documentation/pom.xml
	extensions/completeness-analysis-wizard/pom.xml
	extensions/copy-data-wizard/pom.xml
	extensions/csv-datastore-wizard/pom.xml
	extensions/excel-datastore-wizard/pom.xml
	extensions/jdbc-datastore-wizards/pom.xml
	extensions/network-tools/pom.xml
	extensions/pentaho-job-engine/pom.xml
	extensions/phonetic/pom.xml
	extensions/pom.xml
	extensions/quick-analysis-wizard/pom.xml
	extensions/salesforce-datastore-wizard/pom.xml
	extensions/sample/pom.xml
	extensions/sugarcrm-datastore-wizard/pom.xml
	extensions/test-tools/pom.xml
	extensions/visualization/pom.xml
	extensions/visualization/src/test/scala/org/eobjects/datacleaner/visualization/DensityAnalyzerResultSwingRendererTestApp.scala
	extensions/visualization/src/test/scala/org/eobjects/datacleaner/visualization/StackedAreaAnalyzerResultSwingRendererTestApp.scala
	monitor/api/pom.xml
	monitor/pom.xml
	monitor/services/pom.xml
	monitor/ui/pom.xml
	monitor/widgets/pom.xml
	pom.xml