Relative paths ("../" or "~/") don't work in data_directory config

```
What steps will reproduce the problem?
1. Change the data_directory configuration value to "../src/"
2. Initialize the contest and try to download an input
3. Server returns a 302 error and no input is downloaded

What is the expected output? What do you see instead?
Expected to have the input file downloaded properly. Instead, ServerError
occurs, see the full output in the addl information section below

What version of the product are you using? On what operating system?
1.2 beta1
OSX 10.9.2

Please provide any additional information below.

Full shell output:
codejam-commandline-1.2-beta1 $ python2 gcj_download_input.py A small 0
-------------------------------------------------------------------------------
Small input for "A - Magic Trick" at "../src/2014/qual/A-small-0.in"
-------------------------------------------------------------------------------
Getting status of contest 2974486 from "code.google.com"...
200 OK, 584 bytes read from server (1017 uncompressed).
Getting user status at contest 2974486 from "code.google.com"...
200 OK, 103 bytes read from server (169 uncompressed).
Downloading practice input file.
Getting input file "../src/2014/qual/A-small-0.in" from "code.google.com"...
302 Found, 578 bytes read from server.
ServerError: Error while communicating with the server, cannot download input.
Check that the host, username and contest id are valid.

```

Original issue reported on code.google.com by `matted...@gmail.com` on 25 Apr 2014 at 3:16

Start by locating the data_directory configuration handling and the download path used by gcj_download_input.py. Reproduce the issue with the shown command and a relative data_directory; done means ../ and ~/ paths resolve correctly and the input downloads without ServerError.

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