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
merged branch literal/v1.8.0 (PR #723)

Commits
-------

d13f742 fix example in doc for embed tag
b11412e improve documentation for embed tag

Discussion
----------

improve documentation for embed tag

https://github.com/fabpot/Twig/issues/722

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by fabpot at 2012-05-10T05:10:37Z

I think this is a great improvement over the original version! The notion of "micro-layouts" certainly helps understand the concept behind the `embed` tag.

Thanks.
Coding style

- Unnecessary return in Lexer
- Typo in TokenParserBrokerInterface
merged branch Fustrate/master (PR #725)

Commits
-------

6c06a32 Coding style

Discussion
----------

Minor typos and coding style

- Unnecessary return in Lexer
- Typo in TokenParserBrokerInterface
switched to use mbstring whenever possible instead of iconv (because of issues with iconv and Japanese -- thanks @ganchiku for the tip)
removed obsolete code
fixed a regression introduced by aa6b8358161e4ef35299be06a5ff61bfcba46f6a
fixed CHANGELOG