Version 0.2.5

I previously extended the Etcd3Exception constructor signature such
that it was no longer possible for the derived exception types to
construct with no args.  This change makes that possible again, while
still allowing an Etcd3Exception to be constructed with detail text as
well as the traditional "Bad Request" message.

Whilst working on the Reproducible Builds effort [0] we noticed
that python-etcd3gw could not be built reproducibly.

This is because one of the function signatures used a non-
deterministic / random default value that was rendered by the
documentation system at build time and thus varied each time.

Patch attached.

 [0] https://reproducible-builds.org/

Note: The original author of this patch is Chris Lamb <lamby@debian.org>

    reproducible (Closes: #934918).

utils._decode() returns base64-decoded data. In Python 2 it is an
instance of str() while in Python 3 the returned value is an instance of
bytes(). This commit makes etcd3gw lock work and the unit tests run
correctly without the implicit convention between str and bytes in
Python 3.

Fixes: #24

Causes failure in Tooz. Reverting for now:
https://github.com/dims/etcd3-gateway/commit/224f40972b42c4ff16234c0e78ea765e3fe1af95

This reverts commit 224f4097.

utils._decode should return an instance of 'str' instead of 'bytes' in
Python 3.

Fixes: #24

Locks can not be freed after multiple acquire functions are executed in single case mode

Fixes #18

The previous technique used to try to force-close the
watch socket didn't work on (at least) Python 2.7.12 but the
old Python 3.x workaround seems to work fine on Python 2.7 as
well. Use that.

Fixes #15

The watch API cannot work properly. When you set up a key prefix,
it will still watch the whole key space. This commit fixes it.

The reason is that when it deals with the original key prefix string,
it double encodes the string in client.py and watch.py.

 Add the option to use binary data (e.g msgpack)
in the put/get methods.

Fixes the test_sequential_watch_prefix_once functional test under
python 3.x

Adds a "create" method to the client, which verifies
that the key did not exists in the key-value store,
before assigning the new value to the key

Update etcd version, add py36

Also remove the upper constraints