Support esp-idf master, add .travis.yml, import an example

[trombik]

This PR supports esp-idf master.

snprintf(3) usages in the code have been replaced with a common idiom. there are other possible sources of bugs, but tests should be added first. you may drop 57d1226.

• remove submodules in esp32-homie-example (not needed any more)
• the build matrix includes esp-idf versions and build methods
• check return value of snprintf(3)

[lukecyca]

Thank you very much for this contribution!

• The changes to homie.[ch] look great to me! Do you know what versions of IDF to which this maintains backwards compatibility?

• We had the esp32-homie-example in another repo, using a git submodule to reference the actual library. Is this not a good approach? Have you essentially moved that into this repo such that we should delete that other repo?

• What do I need to do to enable travis on this repo? Just enable a free tier and turn it on? We use Gitlab and its integrated CI for most of our internal projects (only posting public stuff to Github) so I am not as familiar with how best to do CI on github these days.

[trombik]

the supported esp-idf versions should be the latest stable 3.x (3.3 at the moment), and master.

having another repository for examples is not a good idea because maintaining multiple repositories is PITA. examples should evolve as the development progresses. tests and code should be in a single repository. yes, i removed the submodule.

you need to purchase the Open Source plan, which costs zero. go to https://github.com/settings/installations to configure it. set "All repositories" in "Repository access", if you would like to enable travis CI in all your public repositories.

I am working on homie 4 support at https://github.com/trombik/esp32-homie/tree/homie4 now. it is not a trivial change. in that branch, homie 2.x support has been dropped because I cannot test 2.x (openhab2 does not support 2.x any more). some issues are remaining (see issues).