README.md & config.py: Deprecate COBOT_PREFIX

Deprecate the use of the environment variable
COBOT_PREFIX -> BOT_PREFIX
issue warning if COBOT_PREFIX is still being used.

Closes coala#370

README.md

@@ -31,7 +31,7 @@ Read more about what `corobo` could do for you
 ## Environment Variables
 
 1. `COBOT_ROOT` - absolute path of the project root.
-2. `COBOT_PREFIX` - prefix to use for issuing bot commands.
+2. `BOT_PREFIX` - prefix to use for issuing bot commands.
 3. `BOT_ADMINS` - Admins of the errbot instance.
 4. `ROOMS` - Space separated list of rooms to join on startup. e.g.
    `ROOMS="coala/coala coala/coala/corobo"`

config.py

@@ -52,10 +52,16 @@
 BOT_LOG_FILE = os.path.join(COBOT_ROOT, 'errbot.log')
 BOT_LOG_LEVEL = logging.DEBUG
 
-BOT_PREFIX = os.environ.get('COBOT_PREFIX', 'corobo ')
+BOT_PREFIX = os.environ.get('BOT_PREFIX', 'corobo ')
+
+if 'COBOT_PREFIX' in os.environ:
+    BOT_PREFIX = os.environ['COBOT_PREFIX']
+    logging.warning(
+        'Deprecation warning: environment variable COBOT_PREFIX is replaced '
+        'by BOT_PREFIX.')
 
 # Also listen to cobot, if the bot being ran is corobo
-if not os.environ.get('COBOT_PREFIX'):
+if not os.environ.get('BOT_PREFIX'):
     BOT_ALT_PREFIXES = ('cobot ', )
 
 BOT_DEPRECATED_PREFIXES = os.environ.get(