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

[mbektchiev]

• [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

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• 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.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

refs #1196