Made number_format behaviour on null argument compatible with Twig
#245 - Initial support for expression-based inclusions
Update .travis.yml
Merge pull request #254 from lyncodev/null-safe-number-format-250-master

[master] Made number_format behaviour on null argument compatible with Twig
Merge pull request #256 from thomas-p-wilson/fix-include-expression

#245 - Initial support for expression-based inclusions
fix semantic error for checking on double type
check for null and add some tests
Update README.md
Merge pull request #260 from newsletter2go/math_ops_enhancement

fix semantic error for checking on double type