Allow Configuration.MetadataDocumentUri to use relative paths

[JohnLBevan]

Currently the example code has MetadataDocumentUri pointing to a URI.
However, in most scenarios a URI cannot be used; since authenticatoin would be required.
In such scenarios, the solution allows a local filename to be specified. However, currently it requires that this metadata file be specified as an absolute path. This will cause issues where multiple developers are working with the same solution, but have different paths for their repositories.

To resolve this, I propose adding a new property, ProjectRelativeMetadataDocumentUri. When MetadataDocumentUri has a value this new property will be ignored. However, when ProjectRelativeMetadataDocumentUri is null the new property can be used. Host.ResolvePath(ProjectRelativeMetadataDocumentUri) can then be used when importing the configuration in ApplyParametersFromConfigurationClass() to resolve the relative path to an absolute one.

[JohnLBevan]

Closing as duplicate of #77