[#905] Added extent validation and its test.
[#905] Changed the bounding box's text to west, east, north, and south.
Merge pull request #908 from akbargumbira/inasafe

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- Added extent validation on OSM Downloader
- Changed the bounding boxs text to west, east, north, and south instead of minimum latitude, maximum latitude, minimum longitude, and maximum longitude

- Tested by using it interactively on Linux Mint
- Unittests on safe_qgis.tools.test_osm_downloader are all passing on Linux Mint