Add documentation for datasources

Documentation taken from python API documentation, source code, and
reading through issues
add adjust option to text-horizontal-alignment - refs https://github.com/mapnik/mapnik/pull/2426
Merge pull request #77 from mapycz/add-text-horizontal-alignment-adjust

add adjust option to text-horizontal-alignment
new expression property
Merge pull request #64 from pnorman/missing_datasource_properties

Add documentation for datasources
fixes to datasource details
Add docs for raster-colorizer - refs #74
tests/linting quiet by default
fill out default values in the 2.0.0 ref
sync changes to 2.0.1 and 2.0.2 ref
report version of lint warnings
report numbers as json numbers
clean up last remaining unused 'api' properties
fill out more default values
more default values
fill out remaining default values
simplify linting code
port symbolizer linting to js script only
Add some missing default meanings for latest ref
default-meaning + doc cleanups