Add HTTPResponse.tell() as per #276
_wire_bytes_read => _fp_bytes_read
Tweak HTTPResponse.tell() __doc__
Add test_deflate_streaming_tell_intermediate_point()

This (somewhat large) test ensures that tell() returns the correct
values when streaming a large compressed payload.
Fix test for Python3
Merge pull request #277 from pwaller/tell

Add HTTPResponse.tell() as per #276