fixed Markdown::process() to make it compatible with parent declaration.

Control statements based on the if and elseif constructs must have a single space before the opening parenthesis of the conditional and a single space after the closing parenthesis.

- cleanup API and usage, add documetation link
- introduced @property feature in getter or setter which has precedence
  over @return or @param tag if present

fixes #822

to avoid producing different results on different file systems
-> better diffs

See https://github.com/yiisoft/yii2/issues/123#issuecomment-17657374
Issue #123