Fixes issue#1

[nicfix]

When a cyclic dependency is detected method run_tasks raises a CircularDependencyException and prevents tasks execution.

Added a couple of things on test.py:

• Method thread_safe_check_circular_dependencies to recursively check dependencies starting from a certain task
• CircularDependencyException class to raise an error when circular dependencies happen

Modified run.py:

• Added the thread_safe_check_circular_dependencies step before calculating dependencies

Added test_cyclic_dependency.py:

• One test that doesn't causes CircularDependencyException (diamond configuration)
• One test that causes CircularDependencyException

Also added a .gitignore file generated with PyCharm

[nicfix]

I've fixed previous commits problems.