pages: HTTP cache headers handling: when checkCache is used, HTTP headers handling cache are now used: - ETag is first checked, using a hash of the rendered content - Last-Modified is used as a fallback is client is not handling ETag When suitable, a HTTP 304 code (Not Modified) wihtout body is returned instead of the whole page.

try:
    from common.constants import Const as C
except ImportError:
    pass # import doesn't work at this stage with pyjamas
else:
    __version__ = C.APP_VERSION