Possible fix for #474

One way to fix #474 is to have `AutoMapper.targets` only include relative paths in `@(Content)` via `MakeRelative` [property function](http://msdn.microsoft.com/en-us/library/dd633440.aspx#BKMK_MakeRelative), however they were introduced in .NET 4 and unavailable to older versions of .NET/MSBuild and I can't tell if `[MSBuild]` static class is present in other builder engines, i.e. Mono, WP8/Android, etc.