added support for checking out svn revisions to download_file

This works for easyconfig files looking like this (if pysvn and tar are available on the system):


```
name = 'FDS'
version = '20564'

homepage = 'https://code.google.com/p/fds-smv/'
description = """
 Fire Dynamics Simulator (FDS) is a large-eddy simulation (LES) code for low-speed flows, with an emphasis on smoke and
 heat transport from fires.
"""
toolchain = {'name': 'foss', 'version': '2014b'}
toolchainopts = {'pic': True, 'usempi': True}

source_urls = ['svn+http://fds-smv.googlecode.com/svn/trunk/FDS/trunk/']

sources = ['%(version)s.tar.gz']
moduleclass = 'phys'
```

note the 'svn+http' part in the source_urls, this is needed to tell the download function to use svn. version is used as the revision number to check out, 
sources are currently always '%(version)s.tar.gz'
Sine this code checks out the svn repo at the given revision, creates a tarball, and removes it again.

I was doing tests and was frustrated about not knowing how long the download was going to take, I think it's a nice feature, and some people have actually asked me for it...
reporting every 10sec's at the moment, this means it usually won't show up at all, only when the download is slow, and then it's nice to have some kind of indication...