Feature/broadcast rtmp bitrate

[SecondeJK]

This PR adds the ability to choose a maximum Bitrate for new Broadcasts.

Motivation and Context

Evolution of OpenTok SDK

How Has This Been Tested?

New test to pull out the bitrate with expected default value has been added.

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 #339 (69fce3b) into dev (f1e40f8) will increase coverage by 1.8%.
Report is 12 commits behind head on dev.
The diff coverage is 85.3%.

Files Changed	Coverage Δ
src/OpenTok/Util/Validators.php	82.3% <60.0%> (+6.3%)	⬆️
src/OpenTok/Broadcast.php	82.1% <100.0%> (+11.3%)	⬆️
src/OpenTok/OpenTok.php	91.7% <100.0%> (+<0.1%)	⬆️