Implement all parsing methods for Markdown syntax, such that all test
requirements are satisfied.

The commit provides a functioning CI and therefore closes #1.

Adds unittests for DoxecSyntax subclass Markdown. The implementation of these
tests lead to slight modifications in the doxec package. The DoxecSyntax.parse
expects a list of lines, which is modified in place, instead of a plain string.

Create an empty package named doxec.tests to store all unittests.

Fix mistakes indicated by previous pipeline. The errors arise from missing
imports or plain syntax errors.

Add gitlab-ci configuration using the attached docker image using ubuntu 16 and
python3. The CI consists of two test jobs: unittest and doctest

Add the `.gitignore` file containing wildcards for temporary vim files and the
python cache directory.

Adds skeleton classes for dexec package. Most classes define only a list of
methods, without implementation, but a short comment about its purpose. As of
now, the list of classes and methods, seems to be sufficient. The package lacks
unittests and the CI setup.