Refactored execute permission into one spot.

A fixed (0755) execute permission is used in two places, I refactored it into
one method. Bonus: this makes it easy to test the fix proposed in
https://github.com/buildout/buildout/pull/18
Added test whether scripts are made executable.

Note: includes if/else for windows.
Uses http://docs.python.org/2/library/os#os.access
Using distribute's way of chmod +x.

os.umask is a bit weird in that it *does* return the current umask,
but it has a required argument that it immediately sets as the new umask...
Weird. Distribute solves that by assigning a dummy value and re-setting
the umask to the original value.

The matching test verifies that it actually works.

Note that I use python 2.6+'s octal notation: 0o755.
Buildout 2 doesn't support python 2.5 anymore, iirc, so that's fine.
Merge pull request #75 from reinout/use-umask

Use umask when doing "chmod +x". Improved version of pull request #18