Added missing System.Core reference to FSharp.Data.

These references are necessary in order to correctly build FSharp.Data
on Mono/Xamarin Studio.
Added file:// uri scheme guard to isWeb

Fixes incorrect behaviour for absolute paths in Mac/Mono
Merge pull request #130 from jamesholwell/master

Added guard to file resolver to support Mac absolute paths
Merge pull request #128 from diegofrata/master

Added missing System.Core reference to the projects.