fix default comparison in AbstractFilter, cause Propel ORM need only NULL and don't accept empty string
some tests with empty comparison based on NumberFilter
Merge pull request #48 from sidorovich/fix_default_comparison

fix default comparison in AbstractFilter, cause Propel ORM need NUL...
Fixed tests
[documentation] improve Sphinx support
Added a missing travis-ci instruction to install sphinx
[documentation] Fix missing resource in the index