Globber Relative Path Fix
Fixes issue in Cake.Core.IO.Globber.Walk() resulting some relative
paths would result in below exception
 startIndex cannot be larger than length of string.

This because the relative rootpath could be longer/different vs.
child file/folder paths.

I.e. this would fail:
..\cake\src\Cake\bin\Debug\Cake.exe .\build.csx -solution="..\Cake\src\Cake.sln"

Fix uses the PathCollapser.Collapse() to clean up the full root path
causing the issue.
Merge https://github.com/cake-build/cake into globberfix
Merge pull request #104 from devlead/globberfix

Globber relative path fix.