Add possibility to get authentication information from global config

This commit adds a funciton read_global_config() that gets called
whenever the first ArtifactoryPath object is constructed. Then
it reads and parses configuration file ~/.artifactory_python.cfg,
which has the following structure:

[https://artifactory-instance.local/artifactory]
username = foo
password = @dmin
verify = false
cert = ~/path-to-cert

The configuration reading funcitons were generalized, since soon
a new class will be added, 'Artifactory' that will need those too.

In context of #5
Fix a problem where default config doesn't exist

In this case the previous code has thrown an excaption
Fix unittests in python 3
Update README.md to explain ~/.artifactory_python.cfg
Bump version to 0.1.8