Implement raster source feature to read data from third-party sources, to be used in lua profiles.

* Adds a data structure, RasterSource, to store parsed + queryable data
* Adds bindings for that and relevant data structures as well as source_function and segment_function
* Adds relevant unit tests and cucumber tests
* Bring-your-own-data feature

features/step_definitions/data.rb

@@ -134,6 +134,12 @@ Given /^the input file ([^"]*)$/ do |file|
   @osm_str = File.read file
 end
 
+Given /^the raster source$/ do |data|
+  Dir.chdir TEST_FOLDER do
+    File.open("rastersource.asc", "w") {|f| f.write(data)}
+  end
+end
+
 Given /^the data has been saved to disk$/ do
   begin
     write_input_data