Feature: Enable Access to Parent and root, not only to "it"!

Example for a DistinctBy!
Enable Access to "parent" and "root" not only "it"

DistinctBy as Example
Merge branch 'master' of https://github.com/jogibear9988/System.Linq.Dynamic

Conflicts:
	Src/System.Linq.Dynamic.Tests/ComplexTests.cs
Merge branch 'master' of https://github.com/jogibear9988/System.Linq.Dynamic into jogibear
Added symbol equivalents for the context keywords: it, parent and root ($, ^, ~, respetively).
Also added ability in GlobalConfig to disable the standard context
keywords.
Updated docs with new context keywords/symbols and added support for
keywords
Adjusted some webdoc verbage
Fixed unit test, and fixed error handing for parent/root when out of
scope.
Release 1.1.13