Remove curl dependency by using python's urllib2

[lilac]

As far as I know, currently rustc depend on curl for downloading files.

$grep curl src/etc/*
src/etc/snapshot.py:    returncode = subprocess.call(["curl", "-o", tmpf, u])
Binary file src/etc/snapshot.pyc matches
src/etc/unicode.py:        os.system("curl -O http://www.unicode.org/Public/UNIDATA/%s"

However, we can actually remove this dependency by python's owner power.

[brson]

Cargo also has a runtime dependency on curl for certain download strategies, but I'd consider that an independent issue, and if we removed curl from etc. then we could make it not a build dependency.

[lilac]

@brson I change the code to use urllib2, but I later found out that there is not download progress indicator when using this. Thus keep current dependency on curl is reasonable.