Add version to outgoing messages

- Added constant for version of release in constants.py
- Added 'v' value representing node's version number in send messages

This version number will help us see what version nodes are running on when diagnosing issues in the future.
Merge pull request #667 from hoffmabc/master

[CRITICAL] Add version to outgoing messages
Add timeout to resolve row lock bug

Simultaneous access to a row when writing and reading will cause an exception. This allows the proces to wait until it's available and then write instead.
Create only one zmq context to reduce overhead

Creating a zmq context for each PeerConnection object is wrong and creates a massive overhead.

Move context creation to parent class
Remove redundant context creation
Fix executable bit check to work on Mac OS X.

* Ignore results from virtualenv folder.
Merge pull request #672 from Renelvon/mac_checkstyle

Fix executable bit check to work on Mac OS X.
Merge pull request #670 from hoffmabc/master

Bug Fixes (single zmq context, timeout in database connection)
update brew installer url
Reduce threading in market.py

* These operations end up accessing the DB but pySQLite doesn't
  support multithreaded access.
Merge pull request #708 from Renelvon/less_threading

Reduce threading in market.py
Do not use string_types from six.

* No reason to fake Python3 compatibility in just one module.
Remove six from requirements.

* Any package that requires it should install it on its own.
Beta 2.0 release
Merge branch 'master' into develop

Conflicts:
	node/connection.py
	node/market.py
	node/transport.py

* Conflicts in connection.py and transport.py due to
  listener refactor at #678. In order to ensure the instantiation
  of a single zmq Context, the initializers of the listeners have been
  expanded with an additional `ctx` argument.
* Conflicts in market.py due to style fixes in #658.
Test keyInRange against unicode.

* Fix the naming convention in the other tests.
Merge pull request #711 from Renelvon/no_six

Drop six from requirements
Merge pull request #712 from Renelvon/backmerge

Backmerging master into develop
Flag implicit relative imports (W0403).
State relative imports explicitly.

* Reorder imports in accordance to PEP8.
* Replace calls to privtopub with calls to
  privkey_to_pubkey for consistency.