Fixes UTF-16 encoded .addin manifests fail upon loading (#70)

[spitznagel]

RevitTestFramework fails to Run If an Addin manifest is encoded in UTF-16. When run from the console using RevitTestFrameworkConsole.exe, the error message is "There is no Unicode byte order mark. Cannot switch to Unicode." There is no error message shown in the GUI, but it still fails.

We can solve this problem by using StreamReader to automatically detect the encoding of the .addin manifest. Once loaded, we can pass the StreamReader directly to XmlDocument. This works for both UTF-8 and UTF-16 manifests.

[marchello2000]

Thank you, @spitznagel : this has been incorporated into https://github.com/upcodes/RevitTestFramework