Addressing cross-platform installation issues

Attempted to address various cross-platform installation issues:

- Removed all hard-coded references to file-paths, and replaced them
  with paths provided by `appdirs`.

- Removed (erroneously inserted) default file paths which would never be
  checked due to application logic.

- Modified `setup.py` to no longer install a configuration file. The
  mechanics are still in place to **read** a configuration file, but
  after examination, I've concluded that the installation of that file
  is best left to package maintainers rather than `cheat` itself.

cheat/configuration.py

@@ -11,12 +11,10 @@ class Configuration:
         config_file_path_global = self._select([
             os.environ.get('CHEAT_GLOBAL_CONF_PATH'),
             appdirs.site_config_dir('cheat', 'cheat'),
-            '/etc/cheat',
         ])
         config_file_path_local = self._select([
             os.environ.get('CHEAT_LOCAL_CONF_PATH'),
             appdirs.user_config_dir('cheat', 'cheat'),
-            os.path.expanduser('~/.config/cheat/cheat'),
         ])
 
         # attempt to read the global config file
@@ -56,7 +54,6 @@ class Configuration:
             os.environ.get('CHEAT_USER_DIR'),
             os.environ.get('CHEAT_DEFAULT_DIR'),
             os.environ.get('DEFAULT_CHEAT_DIR'),
-            # TODO: XDG home?
             os.path.expanduser(
                 os.path.expandvars(os.path.join('~', '.cheat'))
                 ),
@@ -86,8 +83,6 @@ class Configuration:
             os.environ.get('CHEATPATH'),
             config.get('CHEAT_PATH'),
             appdirs.user_data_dir('cheat', 'cheat'),
-            appdirs.site_data_dir('cheat', 'cheat'),
-            '/usr/share/cheat',
         ])
 
     def _read_config_file(self, path):

setup.py

@@ -1,4 +1,4 @@
-from cheat.appdirs import user_config_dir, user_data_dir
+from cheat.appdirs import user_data_dir
 from distutils.core import setup
 import os
 
@@ -7,11 +7,6 @@ import os
 cheat_path = os.environ.get('CHEAT_PATH') or \
                 user_data_dir('cheat', 'cheat')
 
-# determine the path in which to install the config file
-config_path = os.environ.get('CHEAT_GLOBAL_CONF_PATH') or \
-                os.environ.get('CHEAT_LOCAL_CONF_PATH') or \
-                user_config_dir('cheat', 'cheat')
-
 # aggregate the systme-wide cheatsheets
 cheat_files = []
 for f in os.listdir('cheat/cheatsheets/'):
@@ -41,6 +36,5 @@ setup(
     ],
     data_files=[
         (cheat_path, cheat_files),
-        (config_path, ['config/cheat']),
     ],
 )