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Add dependency to the Salary DSL

Salary DSL intends to be a configuration DSL to specify salary
categories. Not used though yet.
Allow for optional values on cells

This allow to have empty values even when specifying accepted values.
Refactor grammar

Changes:
    * Add Row range to analize on excel sheets.
    * Fix bug when generating user defined rules.
    * Assign name to literal variables.
    * Fix bug on context variable names generation. Need also the sheet name on the variable array
    * Improve error messages with the use of the context variable.
    * Split rule in two categories Static and Iterables.
    * Method getRules() was deprecated in favor of the above two categories.
    * Use of range to evaluate rules in chunks for performance.
    * getContext() method renamed to buildContext().