convert stringex plugin to proper gem

Change-Id: I947b3dee5adc0ac84ef56173c46a1927ca3f2936
Reviewed-on: https://gerrit.instructure.com/29469
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Stephan Hagemann <stephan@pivotallabs.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>