Merge pull request #664 from ka8725/master

Replace stub! with stub in examples of tests
Fix syntax typo
Merge pull request #666 from stuartchaney/patch-1

Fix syntax typo
fix a small typo in README.md
Merge pull request #670 from qqshfox/patch-1

fix a small typo in README.md
allow required param with predefined set of values to be nil,
fixed #635
fix syntax in README.md

The `requires` statement should be in a `params` block according to #560.
Merge pull request #671 from dm1try/fix_required_params_inside_optional_group

allow required param with predefined set of values to be nil inside optional group
Merge pull request #672 from qqshfox/patch-2

fix syntax in README.md
Avoid requiring non-existent fields when using Grape::Entity documentation
Merge pull request #673 from qqshfox/patch-3

Avoid requiring non-existent fields when using Grape::Entity documentation
Preparing for release, 0.8.0.
Preparing for next development iteration, 0.8.1.
Virtus is only for test env