Fix IllegalArgumentException on Windows when using UNC paths

When using java.io.File constructor with an invalid URI, e.g.

    new java.io.File(new java.net.URI("file://c:/activator"))

an `IllegalArgumentException` is thrown with message `URI has an authority component`
This is fixed by replacing changing the file path to use `file:/` (one slash)
More info
[Eclipse/UNC Paths](http://wiki.eclipse.org/Eclipse/UNC_Paths)
[Java mishandles UNC paths on Windows](http://www.tomergabel.com/JavaMishandlesUNCPathsOnWindows.aspx)
Fixes #648