Add optional unit of measurement

[fabaff]

Description:

With the new optional configuration variable unit_of_measurement the sensor can be used to simulate a real sensor.

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#2337

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: random
    name: Temperature
    min: 20
    max: 25
    unit_of_measurement: '°C'

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @pvizeli to be a potential reviewer.