Move and refactor transport tests.

* Move transport tests to test folder.
* Refactor tests in unittest format; use mocks.
* Improve code style.
Drop run_tests.sh.

* Unit tests should be run by calling `nosetests` on the project home
  directory.
Improve contact tests.

* Factor guid and uri in setUpClass.
* Test __repr__ is implemented and __str__ derived.
Merge pull request #594 from Renelvon/more_tests

Unit test node/transport.py and expand tests of node/contact.py.
Test API of pybitcointools.

* Only test the part of API that is currently used.
Switch from arithmetic to pybicointools.main.
Drop unused code, together with import.
Drop arithmetic tests.
Drop arithmetic module.