Add download-only basic option

Add download-only basic option to permit download without building software.

It can be useful for packages with tons of extensions like R which fails to build because an extension version is not online anymore, it happen frequently.

Example (output was truncated by removing dependencies lines) :
eb --download-only R-3.0.2-goolf-1.4.10.eb -r
== temporary log file in case of crash /tmp/easybuild-DilvLk/easybuild-1iRUpm.log
== Download sources and extensions with dependencies
[*] processing /Softs/easybuild/easybuild/easyconfigs/r/R/R-3.0.2-goolf-1.4.10.eb
ERROR: EasyBuild crashed with an error (at easybuild/python/lib/python2.6/site-packages/easybuild_framework-1.15.0dev-py2.6.egg/easybuild/framework/easyblock.py:562 in obtain_file): Couldn't find file mzR_1.10.0.tar.gz anywhere, and downloading it didn't work either...

By preparing download before building, less time is lost by rebuilding because of stupid wrong url.