Add on_tag_removed trigger to pn532

[romerod]

Description:

Add on_release on_tag_removed trigger to pn532 to get a notification when tag is removed.

Related issue (if applicable): closes esphome/feature-requests#172

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#915

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

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

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @OttoWinter, @jesserockz, mind taking a look at this pull request as its been labeled with an integration (pn532) you are listed as a codeowner for? Thanks!
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[jesserockz]

I like it all apart from the naming of on_released maybe on_tag_removed or something...
I was considering adding this at some point. Thanks

[romerod]

I like it all apart from the naming of on_released maybe on_tag_removed or something...
I was considering adding this at some point. Thanks

Ok, the reasoning was to keep it similar to the binary_sensor, but I agree it's easier to understand like that

[jesserockz]

Thanks, Just needs a docs PR then can be merged in.

[ssieb]

You need to edit the description to include the docs PR.
It also looks you might need to rebase the changes to fix conflicts.

[romerod]

Changed the description, rebased and tested.

[jesserockz]

This will not exactly work as it cannot read_tag_ when there is no tag present at the time.
While it wont crash, there will be errors in the log when it tries to authenticate with the tag.
It will return a blank tag with the uid: new nfc::NfcTag(uid, nfc::MIFARE_CLASSIC)

[romerod]

Thanks @jesserockz for finishing the work.