Adding support for loading external XML files

Adjustments to NodeDataProvider and UmbracoDataContext to accommodate
external XML files...in preparation to support preview mode of Umbraco.
This will rely on (when initializing the UmbracoDataContext) checking if
it's preview mode..and if so supplying the path to the preview mode XML
file.
Create README.md
Add preview support to generated templates

Add preview support to generated templates
Adjustments to template

Fixed issues with unescaped parenthesis.
Update README.md
Adjustments for Preview functionality

Worked with Henri on getting preview functionality code smells out...and
integrated better into the NodeDataProvider as well as the
UmbracoDataContext. Involved fixing the NodeCache to support it.
Merge branch 'master' of https://github.com/StephenPAdams/meramedia.LinqToUmbraco
Merge pull request #21 from StephenPAdams/master

Adding Preview Functionality