Allow users to set spatial references for their layers

[TristanDamron]

Is your feature request related to a problem? Please describe. Tabor is currently limited in that it does not allow users to describe the desired spatial referencing system for their layers.

Describe the solution you'd like A good solution might look something like this:

tabor: 0.3.0
layers:
- name: some_poly
  schema: public
  owner: geocml
  srid: 4326
  fields:
  - name: fid
    type: int
    pk: true
  geometry: polygon

Users would add the ID of their desired spatial reference system per layer.

[TristanDamron]

Just thinking... is spatial_reference a better key than srid?