Lambda Soup 0.6.1

• Support quoted attribute values in selectors (Yann Hamdaoui, #6).
• Add Soup.Infix module containing only the infix operators (Fabian Hemmer, #8).
• Add Soup.siblings.
• Add Soup.rev.
• Install .cmx file for Flambda.
• Improve documentation.
• Coverage analysis, new tests, and various internal improvements.

The next version is likely to introduce breaking changes, so constrain dependencies with < 0.7.0.