Conan: Add CI setup

[Croydon]

This pull request will upload entt Conan recipes automatically when new git tags are getting created.

Todo:

1. Credentials

Please add the following credentials in the Travis UI:

CONAN_UPLOAD: API URL to your Conan repository (most likely Bintray)
CONAN_LOGIN_USERNAME: Your Conan repository username (e.g. Bintray username)
CONAN_PASSWORD Your Conan repository password (e.g. Bintray API key)

2. Merge this pull request

3. After a new release happened: Add Bintray meta data

Go to Bintray entt package and fill in meta data (license, url...).

4. Request inclusion in conan-center on the Bintray entt package site

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This is a sub-set of https://github.com/bincrafters/conan-templates-upstream
If in the future something needs changing it should be first looked into this repository if there are already fixes and if not it would be nice if they could be contributed back :)

[Croydon]

How it can look like:
https://bintray.com/croydon/conan/entt%3Askypjack
https://github.com/Croydon/entt/releases

[coveralls]

Coverage remained the same at 100.0% when pulling a16a52a on Croydon:croydon/conan into 41750c3 on skypjack:master.

[skypjack]

I'm a bit busy, I apologize. I think I'll merge this and create a dedicated tag tomorrow or the day after tomorrow. Thank you for your help so far.