added a new recipe
tweaked doc
refactored the hacking twig doc chapter
renamed a doc file
merged two doc files
fixed markup in the doc
added a missing entry in the CHANGELOG
Added a failing test demonstrating the bug with DateTime and timezones

PHP DateTime objects are mutable so changing the timezone when displaying
it affects the following code whereas it does not occur for other
supported formats.
Fixed the handling of the timezone for DateTime objects
merged branch stof/mutable_datetime (PR #717)

Commits
-------

a23d4fb Fixed the handling of the timezone for DateTime objects
a164469 Added a failing test demonstrating the bug with DateTime and timezones

Discussion
----------

Mutable datetime

PHP DateTime objects are mutable so changing the timezone when displaying
it affects the following code whereas it does not occur for other
supported formats.
updated CHANGELOG
enforced interface when adding tests, filters, functions, and node visitors from extensions

This avoids some hard to find bugs where you use a Twig_Function_Method
for instance for a filter. This is hard to find because the interface is
almost the same but the getSafe() contract is not the same.
clarified usage of the dump function (closes #718)
added tests for bitwise operators
added documentation for the bitwise operators (closes #707)
prepared the 1.8.0 release
bumped version to 1.9.0-DEV
fixed dump function is_safe value when html_errors is not defined in php.ini (closes #720)
improve documentation for embed tag
fix example in doc for embed tag