tox: use py.test as test runner

Default unittest's testrunner is poor and shows useless tracebacks in
case something wrong with import statements or syntax.

In other hand, py.test is modern testrunner with cool output. It's used
often in open source python projects.

Resolves: #24
Merge pull request #25 from ikalnitsky/tox-pytest

tox: use py.test as test runner
Fixes serving of newly added files

This commit handles the issue in serve command which cannot host
new files.

Fixed #19
Serve enhanced for tracking of theme files

This commit enhances serve, so it now tracks the files in user's and
default theme directories.

Fixed #18