Option to NOT TO WAIT for Schedule#execute results

Fixes #367
Added tests for Schedule#execute
Add Net::ReadTimeout to list of RETRYABLE_ERRORS
Fix 1.9.3 usage of Net::ReadTimeout, add Timeout::Error
Fix rubocop issues
Merge branch 'master' of https://github.com/gooddata/gooddata-ruby into schedule-execute-wait-option

Conflicts:
	CHANGELOG.md
Merge branch 'master' of https://github.com/gooddata/gooddata-ruby into schedule-execute-wait-option

Conflicts:
	CHANGELOG.md
Merge pull request #374 from korczis/schedule-execute-wait-option

Option to NOT TO WAIT for Schedule#execute results