clean up open file handles properly

As became apparent through the easybuild-easyconfigs unit tests as soon as the total number of available easyconfigs passed the magic threshold of 1024, lots of file handles are left dangling in the current framework.
This becomes especially clear when using `eb --dep-graph` on the full set of available easyconfigs, for example in combination with Python 2.6 since that triggers a bug (http://bugs.python.org/issue3392), see https://jenkins1.ugent.be/view/EasyBuild/job/easybuild-easyconfigs_unit-test_hpcugent_develop/24/testReport/junit/test.easyconfigs.easyconfigs/EasyConfigTest/test_dep_graph/.

This patch fixes the problems, by properly closing open files for `/proc/cpuinfo` and easyblock log files. Part of the patch is for `vsc/utils/fancylogger.py`, so this should be fixed in https://github.com/hpcugent/vsc-base instead.

A nice way of pinpointing the source of the growing set of dangling open file handles is posted at http://stackoverflow.com/questions/2023608/check-what-files-are-open-in-python.