This PR adds some notable changes.

End-user

• File size support
  When downloading the firmware, SamFetch now sends the max file size, so when downloading the firmware with download clients or with browser, it will now display a progress bar and a percent.

  Old	New

• Resuming download
  Now pausing and resuming download won't re-download the same file again from zero.

Internal

• The Keyholder class has replaced with new Session class along with read-only properties for values that should not be changed.

• CSC list is now served in a Python file instead of a separate JSON file.

• All endpoints are marked as asynchronous.

• Reading the incoming data from Kies servers has moved into a KiesData class, now it is more easy to get a value from data.
  Instead of writing like this: data["FUSMsg"]["FUSBody"]["Put"]["DEVICE_MODEL_DISPLAYNAME"]["Data"]
  It can be typed as: KiesData.body["DEVICE_MODEL_DISPLAYNAME"]

• /api/binary endpoint now handles Range headers (both start-end) and returns Content-Range header.

Closes #5 and #4