Added support for ESRI shape files

Specify shape file with the option --shp_file=/path/to/file.shp
The shape file must be a point layer in EPSG 4326 (that is lat/long, as with projection mercator)
Also added a unit test in test/test_shp_file.py and added the test to travis
Note: python-gdal bindings must be installed to support this option