Make glob expansion respect rooted paths

[goric]

Fixes #136, fixes #199. See also discussion in #118.

As suggested here, this change checks whether the source file glob has a '/' prefix and if so, prepends it to the files matched in ExpandGlob.

Given this setup:

pipeline.AddCssBundle("/css/bundle.css", "css/*.css", "/morecss/**/*.css");

Current markup (paths are always relative):

<link href="css/a.css" rel="stylesheet">
<link href="css/b.css" rel="stylesheet">
<link href="morecss/c.css" rel="stylesheet">
<link href="morecss/foo/d.css" rel="stylesheet">

Markup with this change (paths follow the source pattern):

<link href="css/a.css" rel="stylesheet">
<link href="css/b.css" rel="stylesheet">
<link href="/morecss/c.css" rel="stylesheet">
<link href="/morecss/foo/d.css" rel="stylesheet">

[madskristensen]

Thanks