- DataTypeAttribute updated to work with v7 PropertyEditorAlias-based model
- UmbracoCore.7.0.0 used
- .NET Framework retargeted to v4.5  to use Umbraco v7.0.0
typed access for mixins implemented
added descriptive exception when document type exists in db but not in code
fixing backward compatibility: restored GetPropertyValue methods (https://github.com/spopovic/uSiteBuilder/pull/22#issuecomment-62870483)
introduced MixinBase to be able to add/modify logic there later w/o affecting client code; common properties from MixinBase and DocumentTypeBase moved to a class ContentTypeBase
Mixins are now proxies and virtual properties are lazy-loaded as expected
fixed typo in comment
build of test projects fixed: retargeted to .net 4.5
Merge pull request #22 from VladimirLevchuk/feature/mixin-typed-access

typed access for mixins implemented
Merge pull request #21 from VladimirLevchuk/feature/DataType-sync-v7

This fixes spopovic/uSiteBuilder#15