Add check_create function to check whether a raster can be written using a given driver
Add functions to list the available drivers and test for the presence of a driver
Fix a silly mistake in raster_type_convert
Merge branch 'create_methods'

Conflicts:
	src/GDAL.jl

create_methods branch adds a check_create function
check_create(driver,method) lets you test whether a driver
supports the Create() method (default method=0) or the
CreateCopy() method (default method=1)