Make downloads more robust

[zerebubuth]

An issue that seems to keep happening with GMTED data is that the server will drop the connection mid-file and subsequent requests will get an "empty response" for some period of time. This appears to be some sort of TCP-level rate-limiting, but is currently undetectable to us until we try to build the VRT.

What should happen:

1. Use a more reliable download library (e.g: urlgrabber?)
2. Integrate retries with back-off
3. Check download integrity before calling it done (e.g: run gdalinfo on it)

[zerebubuth]

Fixed in #22.