Implement assetcache module with asset hashing.

Provides a module `assetcache` to support automatic cachebusting of
static web assets, rather than the current approach of incrementing
a version number.

On module load, it walks the file/assets/ dir, computes a CRC32 of
each asset, and provides those hashes via `assetcache_get(path)` at
runtime. The primary consumer of hashes at present is the new `asset`
filter for Jinja2, which accepts a relative asset path and returns the
full path, with cache busting, suitable for final use in the template.

Ex: `{{ 'css/main.css' | asset }}`

files/helpers/assetcache.py

@@ -0,0 +1,20 @@
+import os
+import zlib
+from collections import defaultdict
+
+ASSET_DIR = 'files/assets'
+ASSET_CACHE = defaultdict(lambda: None)
+
+def assetcache_build(asset_dir):
+	for root, dirs, files in os.walk(asset_dir):
+		for fname in files:
+			fpath = root + '/' + fname
+			relpath = fpath[len(asset_dir) + 1:].replace('\\', '/')
+			with open(fpath, 'rb') as f:
+				fhash = zlib.crc32(f.read())
+				ASSET_CACHE[relpath] = '%x' % fhash
+
+def assetcache_get(path):
+	return ASSET_CACHE[path]
+
+assetcache_build(ASSET_DIR)