Add ngeo-exportfeatures directive, used in gmf-drawfeature

[adube]

This PR introduces the ability to export features that are drawn using the gmf-drawfeature directive.

• The ngeo.FeatureHelper is responsible of executing the writing of features and download as well.
• A ngeo-exportfeatures directive is responsible of the UI aspect. It will also be responsible to show a drop-down list if more than one format is defined.
• The ngeoExportFeatureFormats constant will be responsible of defining in what formats we want the exportation to be supported. Defaults to KML only, if not defined.

Todo

• support KML export
• support GPX
• show drop-down menu if more than one format defined
• review
• apply reviewer's corrections
• reproject when writing features

We won't use template instead of jQuery to build menu items, because the menu has to be a sibling of the button. That can't work with pure Angular.

Live demo

• http://adube.github.io/ngeo/gmf-drawfeature-export/examples/contribs/gmf/drawfeature.html

[adube]

@fgravin Ready for review. See also the example.

[fgravin]

if

[fgravin]

Good, see my comments.