Use stripe instead of eventbrite to buy tickets

[jhthorsen]

• Require Mojolicious::Plugin::StripePayment
• Add MCT::Model::ConferenceProduct and ::UserProduct
• Defined more tables in mct.sql related to buying products

[jhthorsen]

@rpcme and @marcusramberg have tested http://test.mojoconf.com/2015/register ans @rpcme says "go".