Simulate System.Web, etc. classes

Issue #1:
Added classes, etc. to simulate functionality normally provided by
the System.Web and System.Collections.Specialized namespaces - which
aren't included in PCL frameworks.

It also appears that something added Xamarin.Android and .IOS build
profiles, which supersede the "Mono___" ones I'd previously created, so
I updated the project build profiles accordingly.
PCL-supported DataAnnotations

Issue #1:
Replaced DescriptionAttribute with DisplayAttribute because the former
is not supported by PCL.
Ref: http://stackoverflow.com/a/18913014/1058872
Test project for portable utilities

Added unit test project for validating the new PCL version of the
library. At the moment, it only contains a test fixture for portable
code providing functionality missing from standard PCL frameworks.
Simulated BCL functionality refactored

Re-organized folder and namespace structure for classes that are
simulating non-PCL-compatible functionality for the following reasons:

1. I need to be able to access both the real classes and the simulated
    ones in the unit test project.
2. The DisplayAttribute I was using before is only supported in .NET
    4.5, and I can only create test projects against 4.0
PCL-compatible HttpException

Added a PCL-compatible implementation of HttpException to the subset
of code providing functionality that is not supported by PCL.
Unit tests for UrlEncode

Added unit tests for implementation of HttpUtility.UrlEncode - proving
that the method does not yet provide same behavior as the BCL
implementation :-(