node/kbucket: Do not circumvent magic methods.

Since Contact implemets __ne__, it is best to always
delegate the entire comparison to the magic method.
Remove duplicate test file.
Groom test_network_util.

* Visually separate right from wrong testcases.
* Add provision for running test without nosetest support.
* Remove execute flag.
Do not comment the obvious.
Refactor pyelliptic tests.

* Rewrite tests as unittests.
* Remove execute flag from file.
* Make imports consistent with rest of code.
Rewrite and expand kbucket tests.

* Factor contact creation.
* Drop empty tearDown.
* Never import classes, only modules.
* Test KBucket init.
* Test len.
* Test all cases of getContact
* Test all cases of getContacts.
* Test all cases of addContact.
* Test all cases of removeContact.
* Test keyInRange.