streaming from build() is broken in 0.3.0
Some client code that was working fine w/ docker-py 0.2.3 now breaks on 0.3.0

A minimal test case looks like:

```
ubuntu@ip-10-77-1-34:/tmp/foo$ cat Dockerfile
FROM stackbrew/ubuntu:13.04
RUN echo "hello"
CMD /bin/bash

ubuntu@ip-10-77-1-34:/tmp/foo$ cat test.py
import docker

doc = docker.Client()
gen = doc.build(path='.', stream=True)
for line in gen:
    print line
```

Executing `test.py` under 0.2.3 builds the image fine.  Under 0.3.0, I get 

```
ubuntu@ip-10-77-1-34:/tmp/foo$ python test.py
{"stream":" ---\\u003e Using cache\\n"}

Traceback (most recent call last):
  File "test.py", line 5, in <module>
    for line in gen:
  File "/usr/local/lib/python2.7/dist-packages/docker/client.py", line 239, in _stream_helper
    size = int(socket.readline(), 16)
ValueError: invalid literal for int() with base 16: ''
```

This is Ubuntu 12.04 w/ Docker 0.9.0

diff --git a/docker/client.py b/docker/client.py
index d8f79c92..38355b09 100644
--- a/docker/client.py
+++ b/docker/client.py
@@ -233,17 +233,8 @@ class Client(requests.Session):
 
     def _stream_helper(self, response):
         """Generator for data coming from a chunked-encoded HTTP response."""
-        socket_fp = self._get_raw_response_socket(response)
-        socket_fp.setblocking(1)
-        socket = socket_fp.makefile()
-        while True:
-            size = int(socket.readline(), 16)
-            if size <= 0:
-                break
-            data = socket.readline()
-            if not data:
-                break
-            yield data
+        for line in response.iter_lines(chunk_size=32):
+            yield line
 
     def _multiplexed_buffer_helper(self, response):
         """A generator of multiplexed data blocks read from a buffered
@@ -349,6 +340,9 @@ class Client(requests.Session):
         else:
             context = utils.tar(path)
 
+        if utils.compare_version('1.8', self._version) >= 0:
+            stream = True
+
         u = self._url('/build')
         params = {
             't': tag,
@@ -372,7 +366,7 @@ class Client(requests.Session):
         if context is not None:
             context.close()
 
-        if stream or utils.compare_version('1.8', self._version) >= 0:
+        if stream:
             return self._stream_helper(response)
         else:
             output = self._result(response)
['docker/client.py']