small refactoring of #15 + tests + docs#16

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vlad-khramov wants to merge 3 commits intoyiisoft:masterfrom

vlad-khramov:func

Commits on Mar 30, 2015

Added support for callable as a source for Twig_Function and Twig_Filter
PatchRanger
committed

Commits on Mar 31, 2015

Callable as a source for Twig_Function and Twig_Filter: making tests passed
PatchRanger
committed

Commits on Apr 1, 2015

small refactoring + tests + docs
vlad-khramov
committed