Flask-boost

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* Use js.yml & css.yml to declare assets.
* Now can build assets via console command ``python manage.py build_assets``.
* Fix bugs & add external paramter & better warning info for urlFor js function.
* Upgrade permission to 0.3.0.
* Upgrade bootstrap to 3.3.4.
* Rm url_prefix when register blueprints.
* Add viewport meta tag to head.
* Add absolute_url_for helper, and inject as jinja2 global.
* Add mkdir_p to helpers.
* Refactor utils.uploadsets.
* Track avatars default image.
* Update color vars in ``utils.less``.

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* Add g as global JavaScript variable.
* Add urlFor as global JavaScript function.
* Add page_vars block to inject JavaScript variables to a page.
* Move rules & permissions to jinja2 globals instead of global context.

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* Add csrf token header for Ajax request.
* Add avatar_url property to User model.
* Update filters.timesince.
* Upgrade bootstrap to 3.3.2.
* Clean codes and comments.

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* Remove Flask-Mail support.
* Upgrade font-awesome to 4.3.0.
* Remove useless configs.
* Add app.production attr.
* Remove no-margin-top css style.
* Enable Sentry only in production mode.
* Add highlight to account.signup & account.signin page.
* Fix typo.

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* Remove fab pull
* Clean codes.

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* Remove Flask-Admin support.
* Add hash to assets url.
* Log render time into HTTP header when the user is admin.