Run driver on dynamic port

[dmmcsantos]

Adds the ability to run webdriver on a specific port.

During our project setup we found out that running tests on multiple devices in parallel using a single driver is an issue, the solution provided in https://github.com/rokudev/automated-channel-testing/tree/master/jsLibrary/multipleDevices didn't work either as we wanted to have the control over our roku test lab farm outside the framework.

Currently we are using docker containers and we are able to spin up as much as we want avoiding driver port collisions, however I believe it's useful to spin up new driver instances on a specific port, not only for parallel execution but also if we just want it to run on a different port.

This PR addresses a solution for it, usage examples:

• ./RokuWebDriver_mac starts the driver on the default port 9000
• ./RokuWebDriver_mac 9001 starts the driver on port 9001
• ./RokuWebDriver_mac invalidport if an invalid port is used, it starts the driver on the default port 9000