Support separate test directories for each package

Some projects keep their test directories in the project's root and others keep them inside their package's directories (see http://time-loop.tumblr.com/post/49578950/where-python-developers-keep-their-test-directories for examples). Pythoscope currently supports only the former - all new test modules are put into a single directory inside a test directory at the project's root.

This blueprint consists of two parts.

First, Pythoscope should properly detect and associate existing test directories and test modules inside them with application modules, regardless of their location. New tests for a given module should be put in the location of its related test module.

Second, it should be made possible for the user to mark where the new test modules should land. Current Pythoscope behavior (single test directory per project) may be kept as a default, and option to use separate test directory for each package should be made available.