Koa compatibility

[SEAPUNK]

I want PCB to have an API and properties as close as reasonable to Koa's, so you can fairly easily use Koa's middleware with PCB.

[SEAPUNK]

Koa properties that we're going to copy:

• env - Environment
• middleware - Array of middleware

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• context - ???

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Our own properties:

• call() - Calls the API
• resources - Resource object.