Slim Query/Table Table: manage variables assignment in VariableStore

Variable set ($X=) was not possible on QueryTable(#91 and #369) and TableTable.

I have found a solution for TableTable (pull request #469) but it was not applicable on QueryTable.

This pull request propose a solution to manage both tables:
I have added a variables map in TestResult returned by expectation.
And do a new instruction 'assign' on SlimClient which set variables in the VariableStore.

This pull request requires a protocol modification, other implementations need to be updated (http://www.fitnesse.org/PlugIns#slim).
If the modification is not done in plugins, variables in theses tables can't be used.

generate here directly the new instruction and store in postEvaluationAssertions