readers: Add 'static' to the base Reader's extensions

Instead of just being a base class, we can use it to parse static
files.  It won't actually do any parsing, but we will get path
metadata extraction from read_file, and this will make the
StaticGenerator implementation simpler.

pelican/readers.py

@@ -44,6 +44,7 @@ METADATA_PROCESSORS = {
 
 class Reader(object):
     enabled = True
+    file_extensions = ['static']
     extensions = None
 
     def __init__(self, settings):
@@ -54,6 +55,12 @@ class Reader(object):
             return METADATA_PROCESSORS[name](value, self.settings)
         return value
 
+    def read(self, source_path):
+        "No-op parser"
+        content = None
+        metadata = {}
+        return content, metadata
+
 
 class _FieldBodyTranslator(HTMLTranslator):
 
@@ -311,7 +318,7 @@ class AsciiDocReader(Reader):
 
 EXTENSIONS = {}
 
-for cls in Reader.__subclasses__():
+for cls in [Reader] + Reader.__subclasses__():
     for ext in cls.file_extensions:
         EXTENSIONS[ext] = cls