Merge pull request #32 from clemens-tolboom/feature/index-file

Thanks
Workaround for #31. Relates to #28 #23.
Test feature branches too
Added tests for class attributes
Added test for static methods and attributes
Fix travis reporting success while actually fail

In https://travis-ci.org/clemens-tolboom/uml-generator-php/jobs/21748109 

> ERROR: composer autoloader not found, run "composer install" or see README for instructions
> The command "phpunit" exited with 0.
> Done. Your build exited with 0.

while no error reported.
Added test for constants. Fixes #37
Added class constant support
First merge support added to json2dot
Now generates merged graph with multiple classes/interfaces
Fixed out-of-memory bug with recursive dependency or multiple classes per file
Modified dot output to follow uml styles for arrows
Code style
Documentation.
Renamed option merge into with-parents.
Add output value to test setup.
Add output value to test setup.