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Added support to access the IIdentity/WindowsPrinciple of the remote side.

Introduced multi-targeting for .Net framework and .Net Standard 2.0, via pr19, thanks Stefan. The library is also now strong-name signed.

This is a minor feature/bug fix release.

• Impersonation was improved to support changing the thread principle.
• Fixed string handling when communicating with the native api.

Adds support to authenticate using provided username/password credentials through the new PasswordCredential class.

The class structure relating to credentials has been changed to support this, however, that introduced a breaking change in the code.

Fixes #6 - a mistake in the exception serialization.

Contains a full implementation of the library plus a simple, working client and server demo.

Contains a few bug fixes from the first release of the library, mostly to improve error reporting.