Windows support

I encountered some compilation errors with the latest "windows" branch from unreleased git-crypt v0.4.
It happens when I updated my forked repo with commit 19ea278a31e58dc99da2301a08d2322fdabd0bf9 (2014-06-03).

So I've made some changes and it now compiles fine on Windows:
- commit 164e0a937ef24c3d8290edcae3825b04698c92e7:
- \- got the content of your repo as of 2014-06-03 (overwritten my files)
- \- repository reorganisation (no source change, juste moved to 'src' directory)
- commit 3c1a3054e96344292ef70af68dcfb7f5556eed9c:
- \- corrected in order to compile using mingw32

Can you merge changes to the upstream repo please?
Thanks a lot.