tests: added Page test

new test: Nette\Database\Table\Selection: Page
Database: fixed getting sequence name if it was renamed [closes nette/nette#1443]
added Nette\Bridges; DI\NetteExtension, Tracy panels and Latte macros moved to Bridges (BC break!)
framework uses Tracy instead of Nette\Diagnostics (BC break)
Typo
__toString: when is called as method, exceptions are thrown
added testing support files
added composer.json
composer.json: minimum-stability=dev