Initial work on column expressions.
Merge branch 'master' into columnexpressions

Resolved conflicts:
	Simple.OData.Client.Core/ICommand.cs
	Simple.OData.Client.Core/ODataClientWithCommand.cs
	Simple.OData.Client.Core/ODataCommand.cs
Implemented support for dynamic column collection expressions.
Renamed FilterExpression to ODataExpression.
Added ODataExpression overloads to For, As and NavigateTo.