Virtual paths and VirtualPathProvider are used in dotless instead of physical file names

- Transient LESS files are supported using VirtualPathProvider;
- LessBundle is derived from StyleBundle;
- LessTransform respects dotless configuration (FileReader and Logger types, debug, other engine settings);
- LessTransform omits less transformation for plain CSS files;
- Less transformation errors properly handled;
- Web application is updated to demonstrate transient files functionality (Bootstrap theme variable.less files and custom BootstrapThemeVirtualPathProvider are added);
- System.Web.Optimization.Less assembly version changed to 1.1.0.0.
Merge branch 'master' of https://github.com/milichev/System.Web.Optimization.Less