Tweaks for PyPI

requirements.txt is now concrete and setup.py is abstract as it should
be.

requirements.txt

@@ -1,3 +1,5 @@
+--index-url https://pypi.python.org/simple/
+
 beautifulsoup4==4.5.1
 docopt==0.6.2
 mutagen==1.35.1

setup.py

@@ -27,13 +27,13 @@ setup(
     keywords=['bandcamp', 'downloader', 'music', 'cli', 'albums', 'dl'],
     packages=find_packages(),
     install_requires=[
-        'beautifulsoup4==4.5.1',
+        'beautifulsoup4',
-        'docopt==0.6.2',
+        'docopt',
-        'mutagen==1.35.1',
+        'mutagen',
-        'ply==3.9',
+        'ply',
-        'requests==2.12.4',
+        'requests',
-        'slimit==0.8.1',
+        'slimit',
-        'unicode-slugify==0.1.3'
+        'unicode-slugify',
     ],
     entry_points={
         'console_scripts': [