Adds support for .NET 4.0

- Replaces extension method with origin call to 'GetCustomAttribute'
- Replaces shortcut call of 'GetValue' with generic one
Accessing contexts with path expressions

- Adds the support for path expressions to access parent contexts
- Adds test to for new path expressions
Merge pull request #5 from crra/master

.NET 4.0 support and fun with parent contexts