Prerequisite changes in GUIDMixin and KBucket.

* Add and test hashing in GUIDMixin; a GUIDMixin can now be used
  as a key in a dictionary.
* Add and test iterator interface in KBucket.
* Add GUIDMixin support in keyInRange.
* Drop default value for market_id and relevant test.
* Test KBucket.addContact upgrades basestrings to GUIDMixins.
* Follow python naming conventions.
General maintainance.

* Update docstrings and function signatures.
* Follow python naming conventions as much as possible.
* Be consistent in the argument names.
* Require all methods to accept GUIDMixin as ID.
* Drop unused TimeoutError class.
* Purge exec flag from routingtable.py.
Setup initial testsuite; refactor abstract methods.

* Make abstract methods raise NotImplementedError; test most of them.
* Add initialization tests for abstract class.
* Follow python naming conventions.
Test and refactor initialization of RoutingTable implementations.

* Initialize superclasses properly.
* Use `super` instead of naming the superclass explicitly.
* Defer testing of other methods for now.
* Follow python naming conventions.
* Delegate logger instantiation to superclass.
Fix and abstract ID range. Test init attributes.

* Install an ad-hoc equality for KBuckets.
* Follow python naming convention.
Test and refactor kbucketIndex.

* Handle GUIDMixin transparently.
* Raise KeyError on key with bad range.
* Raise RuntimeError if kbucket invariant is violated.
* Include a hint for a possible optimization.
* Follow python naming conventions.
Test and refactor getContact.

* Handle GUIDMixin transparently.
* Ignore the docstring mentioning ValueError and
  test for a None value on unsuccessful lookup, as
  is the case throughout the code.
* Align signature with kbucket.GetContact and simplify code.
* Follow python naming convention.
Test and refactor removeContact(s).

* Make argument names consistent with rest of tested code.
* Leverage exceptions to simplify code structure.
* Add logging on exception.
* Follow python naming convention.
Test and refactor RoutingTable.distance.

* Raise ValueError if strings have unequal lengths.
* Handle guid.GUIDMixins transparently.
* Follow python naming conventions.
Test and refactor touchKBucket.

* Add ability to set timestamp manually.
* Test the abstract method, too.
Expand and test constant names.

* Purge exec flag from constants.py
* Define the maximum number of contacts per replacement cache.
* Define HEX_NODE_LEN_ID as the length of a node ID in hex.
Refactor node ID processing.

* Add and test numToId for easy conversions.
* Refactor _randomIDInBucketRange to use numToId.
* Refactor distance to use node ID length, and raise
  an exception when either of the keys is of improper
  length.
Test and refactor getRefreshList.

* Be deterministic; do not use randomized IDs.
* Drop _randomIDInBucketRange as it is no longer used.
* Use a list comprehension for simplicity.
* Avoid invoking time.time() many times. :P
* Optimization: do not enter loop if force flag is set.
* Drop unused import.
Drop addContact from TreeRoutingTable.

* Its proper implementation is contested.
* It references undefined names ("addErrback").
* The class itself is nowhere used in the code.
* It is very difficult to test, as it entails the network.
Merge pull request #730 from Renelvon/refactor_routingtable

Refactoring and unit testing of routingtable (part 1).
Refactor network_util.

* Add leak-safe check_port. Replace special-purpose code.
  * Prefer ipv6 over ipv4 sockets.
  * Accept variable timeout.
* Use tornado.netutil to check for valid IPs.
* Abstract protocol in get_peer_url.
* Prefer importing modules than names.
* Purge exec flag from file.
* Other style fixes.
Refactor code in connection.py.

* Drop unused peer_alive attribute.
* Replace bare except clause.
* Drop unused PeerConnection.responses_received.
* Refactor PeerConnection.create_zmq_socket.
  * Allow it to raise an exception; we are catching it elsewhere.
  * Make it a private function.
* Refactor PeerConnection.send_raw.
  * Make failure cases stand out.
  * Do not compress until ready to send.
* Make failure cases stand out in CryptoPeerConnection.start_handshake.
* Rewrite CryptoPeerConnection.check_port to use network_util.
* Shorten CryptoPeerConnection.sign.
* Straighten CryptoPeerConnection.encrypt.
* Refactor CryptoPeerConnection.send.
  * Make failure cases stand out.
  * Guard against encrypt returning False.
* Remove PeerListener.cleanup_context. PeerListener no longer manages
  it own zmq.Context. Purge any references.
* Drop PeerListener.stop.
  * Do not destroy global context arbitrarily.
* Straighten PeerListener.listen.
* Remove unused import.
Use network_util utilities in DHT.

* Remove comment referring cleanup_context.
Turn TransportLayer.shutdown into abstract method.