redesign steps
added method `getStorage` to `FormFlowInterface`
updated a sentence about validation groups
fixed evaluation of closures for the `skip` option when resetting the flow
updated upgrade guide
fix invalid step index
prefer callable in favor of Closure
Merge pull request #54 from havvg/fix-invalid-steps

fix invalid step index
updated phpdoc for previous commit
added a note about behavioral difference between `getCurrentStep` and `getCurrentStepNumber`
Merge pull request #55 from havvg/prefer-callable

prefer callable in favor of Closure
updated README for previous commit
added `evaluateSkipping` to `StepInterface`, will receive the flow instead of the form data as 2nd argument
add GetStepsEvent
Merge pull request #52 from havvg/wip-redesign-steps

add GetStepsEvent
updated docs and a test for previous commit
Merge remote-tracking branch 'origin/master' into wip-redesign-steps
added some methods for BC with third-party templates
mention GetStepsEvent in the changelog
added a note about the `skip` function being called several times