build: add documentation coverage checks

[dsebastien]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

We don't check for code documentation coverage.

Expected behavior

We should check for documentation coverage at two levels:

• pre-commit (hook)
• Travis CI

We should at least always have >=85% documentation coverage. If we can go higher, all the better indeed ;-)

What is the motivation / use case for changing the behavior?

In order to maintain the quality of the API docs, we need to at least check for completion %.

[Mallikki]

I've done the pre-commit part so far. (#429)

Still need to work on the Travis CI with @christophercr as discussed.

Edit: added! We still need to discuss if the coverage check in .travis.yml is sufficient enough.

Also, the coverage value is now at 85% (was previously set at 70%, the default value)