Bugfixes and support for more payment providers

Bugfixes:

• Fixed issue that caused boot to fail when a database connection was not available (#156)
• Fixed issue related to doctrine/common not being required on PHP 5.3
• Remove unneeded composer dependencies
• Require specific versions of composer dependencies

New payment backends (through third-party bundles):

• Stripe
• Webpay
• YandexKassa