Fix indentation of multiline operator expressions

Expressions with multiple operators over multiple lines got indented one
offset deeper per line break. An additional smie check for being a
sibling should fix this problem.
Merge pull request #61 from ckruse/fix_55

Fix #55: indentation of multiline operator expressions
Add ckruse to the Acknowledgements in README
Change '_' character class to 'symbol constituent'

Resolves 62