deduplicate requirements (NeuralEnsemble#159)

* deduplicate requirements: `setup.py` reads now dependencies from the `requirements.txt` 
*  changed from one requirements file to categorical requirements

MANIFEST.in

@@ -0,0 +1,2 @@
+# Include requirements
+include requirement*.txt

requirements-docs.txt

@@ -0,0 +1,2 @@
+numpydoc>=0.5
+sphinx>=1.2.2

requirements-extras.txt

@@ -0,0 +1,2 @@
+pandas>=0.14.1
+scikit-learn

requirements-tests.txt

@@ -0,0 +1 @@
+nose>=1.3.3

requirements.txt

@@ -1,14 +1,5 @@
-# essential
 neo>=0.5.0
 numpy>=1.8.2
 quantities>=0.10.1
 scipy>=0.14.0
-six>=1.10.0
-# optional
-#pandas>=0.14.1
-# for building documentation
-#numpydoc>=0.5
-#sklearn>=0.15.1
-#sphinx>=1.2.2
-# for running tests
-nose>=1.3.3
+six>=1.10.0

setup.py

@@ -9,15 +9,12 @@
     from urllib import urlretrieve
 
 long_description = open("README.rst").read()
-install_requires = ['neo>=0.5.0',
-                    'numpy>=1.8.2',
-                    'quantities>=0.10.1',
-                    'scipy>=0.14.0',
-                    'six>=1.10.0']
-extras_require = {'pandas': ['pandas>=0.14.1'],
-                  'docs': ['numpydoc>=0.5',
-                           'sphinx>=1.2.2'],
-                  'tests': ['nose>=1.3.3']}
+with open('requirements.txt', 'r') as fp:
+    install_requires = fp.read()
+extras_require = {}
+for extra in ['extras', 'docs', 'tests']:
+    with open('requirements-{0}.txt'.format(extra), 'r') as fp:
+        extras_require[extra] = fp.read()
 
 # spade specific
 is_64bit = sys.maxsize > 2 ** 32
@@ -51,13 +48,14 @@
         os.path.join('spade_src', 'LICENSE'),
         os.path.join('spade_src', '*.so')
     ]},
-    
+
     install_requires=install_requires,
     extras_require=extras_require,
 
     author="Elephant authors and contributors",
     author_email="andrew.davison@unic.cnrs-gif.fr",
-    description="Elephant is a package for analysis of electrophysiology data in Python",
+    description="Elephant is a package for analysis of electrophysiology"
+                " data in Python",
     long_description=long_description,
     license="BSD",
     url='http://neuralensemble.org/elephant',