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.

this actually comes from https://github.com/hpcugent/easybuild-framework/pull/1066

but since it's both touching the same file in the same place I left it in here. Can you review this in #1066