Make ObjectMapper configurable

[FrelliBB]

Since we are using ObjectMapper for JSON de/serialization we should provide a way for Bastion users to configure the ObjectMapper we use for Bastion; ideally through Bastion.config().

Therefore we should also make sure that we use the same instance of ObjectMapper whenever we de/serialize.

I think we can have Bastion.config().objectMapper() provide an instance which can be configured.

[KPull]

There are some left overs of Google's GSON in the code. For anyone tackling this task, it would be great if we remove any references to Google GSON and use Jackson's ObjectMapper instead.