Custom User Agent

[SecondeJK]

This PR adds the ability for developers to create a new Client with a custom app name, to be included in the User-Agent string.

Motivation and Context

This has been requested to be rolled out across all of the SDKs for client tracking support.

How Has This Been Tested?

Tests have been modified to pull out the original static string testing. As the string is dynamic, the new test for this is to only test the method that adds a custom app string to the Client.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #328 (c0c470d) into dev (a1e0533) will increase coverage by 0.1%.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
src/OpenTok/OpenTok.php	91.1% <100.0%> (ø)
src/OpenTok/Util/Client.php	74.9% <100.0%> (+0.4%)	⬆️