Move service extension from boot() to extend() blocks in register()
Improve silex/web-profiler version constraint in README
Declare dependencies on twig and url_generator service providers
merged branch igorw/improvements (PR #2)

This PR was merged into the master branch.

Commits
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917f2b2 Declare dependencies on twig and url_generator service providers
b95de67 Improve silex/web-profiler version constraint in README
697be8e Move service extension from boot() to extend() blocks in register()

Discussion
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Improvements

* Improved version constraint in README
* Noted dependency on twig and url_generator providers in README
* Moved provider logic to `extend()`