[conan] Add header-only style conan support

[Fohlen]

Add's conanfile.py via conan template

[Fohlen]

I just so copied the .github folder and believe it could also be deleted.

[cbbowen]

I'm still learning conan and travis, but it seems like it would be simpler (and less fraught with license complications) to do this without the shell scripts. Something like:

language: cpp
os: linux
dist: trusty
sudo: required
addons:
  apt:
    sources:
      - ubuntu-toolchain-r-test
    packages:
      - g++-8
env: CC=gcc-8 CXX=g++-8
before_install: pip install conan
install: conan user
script: conan create . user/channel

But I'm probably missing something.

[Fohlen]

In theory this could be done. However the xenial and trusty images comes with libstdc5 (or earlier) and it is cumbersome and error prone to upgrade to any more recent version (which is needed for C++17). Using a pre-backed docker with all the tools makes life much easier.

[cbbowen]

I think having Travis set up this way doesn't quite work, because it tries to fetch the latest release archive rather than using the current code, which seems to defeat the purpose of CI. See the travis build log here: https://travis-ci.org/cbbowen/trytravis-scratch/builds/481678825.

[Fohlen]

Yup this seems wrong, I will investigate why it does not conan create . locally, which would be the intended purpose.