Rfxtrx sensor

[Danielhiversen]

Description:
Allow fire event when receiving signal from a Rfxtrx sensor

Related issue (if applicable): fixes some problems mentioned here #2290

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#660

Example entry for configuration.yaml (if applicable):

Checklist:

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

• Documentation added/updated in home-assistant.io

If code communicates with devices, web services, or a:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

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.

[balloob]

Looks good! 🐬