Fixed #16

Sorry for my ugly python code, I will refactor it a bit later.
Some of these fixes should be ported to GitGutter.
Also my drawing abilities are very poor, so if smb can draw a good icon for non-versioned files, it will be great!

Also I noticed performance degradation on huge SVN repos (even local 'svn status <file>' execution can take about 0.5s). But I hope VcsGutter users basically don't use Subversion.