Dot v1.0.1 - interface{} handling

@NV4RE described the unusual behaviour of the dot package: #2

The problem is that if the data insertion path is of type interface{} then dot won't know how to determine the further path.

• type incompatibility bug was fixed if the value was of type interface{};
• error return was added if data insertion path meets interface{} type on its way.

PR: #3