Add JSON dependency to core

[karussell]

As discussed here introducing the JSON dependency in core could make several things less clunky, especially since it seems we need this in #1112 in the core as we use the json format instead of the hacky binary+text mix of StorableProperties. (It will not happen for 0.10)

[karussell]

If JTS is able to read and write geojson on its own we probably don't need to add jackson to the core.

[willcohen]

I should note that JTS also broke itself into a few modules, so jts-core is the main replacement, and then there's also jts-io-common. They use json-simple under the hood.

[karussell]

Thanks for highlighting this! I think we will then better stay with jackson instead of using the inactive json-simple. (conveyal/gtfs-lib#97 (comment))

[karussell]

Really hard decision, but I think for now we close this and try to keep JSON out of the core. Still unsure.