Add support for using variables in abstract patterns

[kosek]

Variable references in abstract patterns are replaced in value attribute as well in order to support let element. Original code wasn't replacing variable references in sch:let/ elements.

[dmj]

There is a test for that bug: https://github.com/dmj/schematron-test/tree/t/pull-57/pattern

[tgraham-antenna]

Fixes same problem as #50.

Related issue: #5