test_2_6: test and changes for 2.6 found by test

- added test for FilesCompleter
- made completers:_wrapcall() python 2.6 compatible
- test.py changes for python 2.6

While testing on 2.6 (with tox) I found some problems with the FilesCompleter which went
unnoticed before: check_output is 2.7 and later. 

I added tests to ``test.py``,  then proceeded to fix. 
-  needed some temporary directory that gets automatically deleted hence TempDir() 
- _wrapcall(), I fixed this by copying the 2.7 check_output code for 2.6 only.
- unittest.skip is > 2.7, did not find an alternative so that whole method is now if sys._version_info >= (2.7)