Adds support for RESTful operations on Styles

[mejackreed]

Fixes #10 by providing a REST client for GeoServer Styles.

As noted in the class comments, the GeoServer Style API can support style creations in individual workspaces and layers, however, this PR only implements them at the top level.

See: https://docs.geoserver.org/latest/en/api/#1.0.0/styles.yaml

README documentation also update:

# via ./bin/console
Geoserver::Publish::Style.new.create(style_name: "raster_layer", filename: "raster_layer.sld")
sld = File.read("./spec/fixtures/files/payload/raster_layer.sld")
Geoserver::Publish::Style.new.update(style_name: "raster_layer", filename: "raster_layer.sld", payload: sld)