generators: Teach Generator.get_files to handle non-directory input

This makes it easier for StaticGenerator to walk FILES_TO_COPY, where
the input may be a directory or a bare filename.

Non-traversable file types (e.g. everything but directories and
symlinks to directories) are not checked against the exclude list.
The user-level effect of this is that explicit entries in STATIC_PATHS
or FILES_TO_COPY will override a hypothetical STATIC_EXCLUDES setting,
which seems like a reasonable approach.

I also removed the Python 2.5 compatibility check for `followlinks` in
os.walk, since Pelican is now Python >=2.7.
endswith can take a list as an argument ;)