v0.2.0

Fix missing requirements & GDAL installation error by @Thomatous in #21
Implement the add_raster() method in the chartops.Map() class by @Thomatous in #17
Implement the add_image() method in the chartops.Map() class by @Thomatous in #18
Implement the add_video() method in the chartops.Map() class by @Thomatous in #20
Implement the add_wms_layer() method in the chartops.Map() class by @Thomatous in #19