[test][is_valid] add as test case the polygon used in the documentation
example for is_valid
[doc][examples] change the color used for drawing the geometry
[doc][examples][is_simple] add image
[doc][examples][is_valid] add image; slightly modify the example geometry
(one of the interior rings was having CW orientation instead of CCW)
[doc][images] add PNG images for is_valid and is_simple documentation
[algorithms][is_valid] remove obsolete comment
Merge pull request #82 from mkaravel/feature/distance

[doc][algorithms] add documentation for the comparable_distance free function that takes a strategy
Merge pull request #81 from mkaravel/feature/is_simple

Feature/is simple
[strategies][distance] implement the class distance_result
[strategies][comparable distance] implement the class
comparable_distance_result
[strategies][distance] re-implement default_distance_result using
distance_result
[strategies][comparable distance] re-implement default_comparable_distance_result
using comparable_distance_result
[algorithms][comparable distance] change the return type of
comparable_distance to comparable_distance_result<...>::type
[algorithms][distance] polish includes; move code in resolve_strategy::result_of
and resolve_variant::result_of namespaces to strategies/distance_result.hpp;
change return type for the distance free function to distance_result<...>::type
or default_distance_result<...>::type (depending on whether a strategy is
passed or not, respectively)