Add MQTT Subscribe sensor

[OttoWinter]

Description:

Related issue (if applicable): fixes

Pull request in esphomedocs with documentation (if applicable): esphome/esphome-docs#50
Pull request in esphomelib with C++ framework changes (if applicable): esphome/esphome-core#193

Example entry for YAML configuration (if applicable):

sensor:
- platform: mqtt_subscribe
  name: "The Value"
  topic: "the/topic"

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).
• Check this box if you have read, understand, comply, and agree with the Code of Conduct.

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

• Documentation added/updated in esphomedocs.