Missing dependency on cython breaks OS X installation
Tested with homebrew python 2.7.9:

```
e106982-mac:~ jamcro01$ pip install pyOCD
Collecting pyOCD
  Using cached pyOCD-0.4.3.zip
Collecting hidapi (from pyOCD)
  Using cached hidapi-0.7.99-5.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 20, in <module>
      File "/private/var/folders/yg/wqy54z_s18ng_3q7qndkbg9nszkzrm/T/pip-build-pz8Kjf/hidapi/setup.py", line 4, in <module>
        from Cython.Distutils import build_ext
    ImportError: No module named Cython.Distutils

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/yg/wqy54z_s18ng_3q7qndkbg9nszkzrm/T/pip-build-pz8Kjf/hidapi
```

Try installing cython:

```
e106982-mac:~ jamcro01$ pip install cython
Collecting cython
  Downloading Cython-0.22-cp27-none-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.macosx_10_10_intel.macosx_10_10_x86_64.whl (3.7MB)
    100% |████████████████████████████████| 3.7MB 120kB/s 
Installing collected packages: cython
Successfully installed cython-0.22
```

Then everything works:

```
e106982-mac:~ jamcro01$ pip install pyOCD
Collecting pyOCD
  Using cached pyOCD-0.4.3.zip
Collecting hidapi (from pyOCD)
  Using cached hidapi-0.7.99-5.tar.gz
Collecting intelhex (from pyOCD)
  Downloading intelhex-2.0-py2.py3-none-any.whl (42kB)
    100% |████████████████████████████████| 45kB 1.5MB/s 
Installing collected packages: hidapi, intelhex, pyOCD
  Running setup.py install for hidapi
  Running setup.py install for pyOCD
Successfully installed hidapi-0.7.99.post5 intelhex-2.0 pyOCD-0.4.3
```

diff --git a/setup.py b/setup.py
index f6e3495d..ab4c1e0e 100644
--- a/setup.py
+++ b/setup.py
@@ -18,6 +18,7 @@
 from setuptools import setup, find_packages
 import sys
 
+setup_requires=['setuptools_scm!=1.5.3,!=1.5.4']
 install_requires = ['intelhex']
 if sys.platform.startswith('linux'):
     install_requires.extend([
@@ -28,6 +29,9 @@ elif sys.platform.startswith('win'):
         'pywinusb',
     ])
 elif sys.platform.startswith('darwin'):
+    setup_requires.extend([
+        'cython',
+    ])
     install_requires.extend([
         'hidapi',
     ])
@@ -38,7 +42,7 @@ setup(
         'local_scheme': 'dirty-tag',
         'write_to': 'pyOCD/_version.py'
     },
-    setup_requires=['setuptools_scm!=1.5.3,!=1.5.4'],
+    setup_requires=setup_requires,
     description="CMSIS-DAP debugger for Python",
     long_description=open('README.rst', 'Ur').read(),
     author="samux, emilmont",
['setup.py']