Automate PyPI release of SkyApi package

**Is your feature request related to a problem? Please describe.**
SkyApi will be part of next release and there are no scripts to automate this process.

**Describe the solution you'd like**
- [x] Build wheel packages in Travis `deploy` (`release-*` branchs only)
- [x] Push to PyPI sandbox.
- [x] Run test suite after downloading and installing aforementioned packages

... once this process succeeds ...

- [x] Build wheel packages in Travis `deploy` (tags only)
- [ ] Push to official PyPI
- [ ] Check packages (manually?)
- [ ] If errors detected then prepare `post` releases to fix'em
- [ ] Document the flow for both `SkyApi` and `PySkycoin` packages

**Describe alternatives you've considered**
Doing this by hand is possible , but not an option ...

**Additional context**
There is a `deploy` section in `.travis.yml` already .