Add Geometry#unary_union
Add feature detection for unary_union to native implementations.

* Add RGeo::Geos::CAPIFactory._supports_unary_union? so client
  code can detect if CAPI supports unary_union.
* Add ffi_supports_unary_union so client code can detect if
  FFI-GEOS supports unary_union.
* Change unary_union tests so they account for the presence of
  unary_union in their implementation.
Merge branch 'unary_union' of github.com:agrismart/rgeo into unary_union

Conflicts:
	test/geos_capi/tc_misc.rb
	test/geos_ffi/tc_misc.rb