fix(marshalling): Expose C strings as references to uint8 in JS

[Marshalling] Change underlying type of returned references from int8 to uint8
[TestFixtures] Use stringWithUTF8String instead of formatting with %s (which uses system default encoding)
[TestRunner] Add utf8 node module to TestRunner project
[TestRunner] Add a test with cyrillic characters which validates that references are indeed unsigned

PR Checklist

[x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
[x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
[x] You have signed the CLA.
[x] All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
[x] Tests for the changes are included.

refs #1196