added user-agent string to requests

@bootstraponline and I decided to just quickly implement adding user-agent strings to selenium/appium clients for easy identification and bug reporting.

The current format:
`author/libraryName/version`

In the future maybe we can add OperatingSystem as well, but we're going for the MVP right now.

see: https://github.com/appium/ruby_lib/commit/52920a4c02701991a5c59a2a41424ad9e22595a5