TravisCI: Enabled raster support
types: Added basic Raster type
test_functional: Added Ocean table

Make sure there are no errors when using the Raster type on PostGIS 2.x
__init__: Create indices for raster columns
Merge remote-tracking branch 'origin/master' into raster

Conflicts:
	.travis.yml
test_functional: Added insertion test for raster column
elements: Added RasterElement class
types: Added result_processor() to Raster type

This transforms the SQL row output into RasterElement instances
functions: Added ST_Height and ST_Width functions
functions: Added ST_Value function
elements: Allow calling ST_ functions on WKTElements
functions: Added ST_AsRaster function
Merge remote-tracking branch 'origin/master' into raster
doc: Added types.Raster documentation
doc: Fixed WKBElement reference in _GISType class
doc: Added elements.RasterElement documentation
doc: Added return type documentation to ST_AsRaster function
Merge branch 'master' into raster

Conflicts:
	geoalchemy2/functions.py
functions: Create raster functions dynamically
functions: Added raster function docstrings