Add ability to merge gym and pokestop json files

[jchristi]

• Adds a command line option -ft to specify "filetype" to be merged. Valid values are 'gyms', 'stops' and 'spawns'. If left empty or any other value is specified then it will merge spawns. When gyms or stops are merged, automatically merge geo_*.json equivalent.
• Converted spaces to tabs in json_to_geojson.py
• Created a convert_to_geojson() method instead of repeating same code
• Added if name == '__main__': for when scripts are run directly