Pickling capabilities for faster start up times. (#252)

On a RaspberryPi it's taking 10 seconds to simply run:

import escpos.printer

This change creates a pickle file that will load 20x faster. The
rationale is that the capabilities.json file doesn't change too often.

Also changed some imports for PEP8.

src/escpos/capabilities.py

@@ -1,16 +1,47 @@
 import re
-import six
 from os import environ, path
+import pickle
+import logging
+import time
+
+import six
 import yaml
 
-# Load external printer database
-if 'ESCPOS_CAPABILITIES_FILE' in environ:
-    file_path = environ['ESCPOS_CAPABILITIES_FILE']
-else:
-    file_path = path.join(path.dirname(__file__), 'capabilities.json')
 
-with open(file_path) as f:
-    CAPABILITIES = yaml.load(f)
+logging.basicConfig()
+logger = logging.getLogger(__name__)
+
+
+pickle_dir = environ.get('ESCPOS_CAPABILITIES_PICKLE_DIR', '/tmp/')
+pickle_path = path.join(pickle_dir, 'capabilities.pickle')
+capabilities_path = environ.get(
+    'ESCPOS_CAPABILITIES_FILE',
+    path.join(path.dirname(__file__), 'capabilities.json'))
+
+# Load external printer database
+t0 = time.time()
+logger.debug('Using capabilities from file: %s', capabilities_path)
+if path.exists(pickle_path):
+    if path.getmtime(capabilities_path) > path.getmtime(pickle_path):
+        logger.debug('Found a more recent capabilities file')
+        full_load = True
+    else:
+        full_load = False
+        logger.debug('Loading capabilities from pickle in %s', pickle_path)
+        with open(pickle_path, 'rb') as cf:
+            CAPABILITIES = pickle.load(cf)
+else:
+    logger.debug('Capabilities pickle file not found: %s', pickle_path)
+    full_load = True
+
+if full_load:
+    logger.debug('Loading and pickling capabilities')
+    with open(capabilities_path) as cp, open(pickle_path, 'wb') as pp:
+        CAPABILITIES = yaml.load(cp)
+        pickle.dump(CAPABILITIES, pp)
+
+logger.debug('Finished loading capabilities took %.2fs', time.time() - t0)
+
 
 PROFILES = CAPABILITIES['profiles']