Add test coverage

[gribok]

Based on #207, I added measuring for test coverage.

See example https://coveralls.io/github/gribok/coronavirus-tracker-api

Features:

• Add section coverage-test to Makefile
  Basically everyone can run locally test cases with coverage measuring via make coverage-test.
  I will create .coverage-sqlite-file, which can be reviewed witch coverage report.

  $ make coverage-test
  [output ottmited]
  $ ls -a .coverage
  .coverage
  $ file .coverage
  .coverage: SQLite 3.x database
  $ coverage report
  Name                                Stmts   Miss  Cover
  -------------------------------------------------------
  [output ottmited]
  -------------------------------------------------------
  TOTAL                                 328     21    94%

• Add coversalls to Travis-CI pipeline to visualize statement coverage in code.
• Add coveralls badge to README
• At every PR, coveralls will show the measuring

ToDos

• Activate coveralls (https://coveralls.io/) from one maintainer
• Thresholding is deactivated by default. It is possible to define threshold via settings of coveralls, if you switch into your branch.
  It makes sense to setup a threshold, that every new code should be merge with covered test cases

Please merge #205 first, to get a clear merge history.

Enjoy!

[gribok]

The after_success step in Travis-CI failed, because the access coveralls is not unlocked.

[Kilo59]

Looks like you've used the same branch for both of these PRs
#205

[Kilo59]

@gribok
I basically did the same thing on my fork as well.
Except I used the pytest-cov plugin instead of coverage directly.
https://coveralls.io/github/Kilo59/coronavirus-tracker-api
master...Kilo59:master

[gribok]

I am going to close the PR, seperate the changes for #169 and #207 in two branches and create new PRs based on mentioned issues.