add support for PostgreSQL JSON type

[goneri]

With this change, it's now possible to push JSON in a JSON field. For example, in this example, the struct file has the type JSON.

POST http://127.0.0.1:5000/notifications
Content-Type: application/json

[{"environment_id":"ae7d4ed8-41b4-0206-6aad-0074c0f5b6f1","struct":{"type":"gerrit","server":"somewhere","account":"bob","port":29418}}]

Without this change, the validator will fail because it don't know about JSON. It will expect a string instead, which is the default type.

This patch requires the following patch: https://github.com/nicolaiarocci/cerberus/pull/79

[amleczko]

I will wait until new cerberus is released.

[goneri]

Hi @amleczko,

I dropped the dependency on a new cerberus release.

[amleczko]

Can you add a simple test coverage, too? Thanks!