memento: use mp_ modifier to support memento with frame or non-frame replay
change memento test to use frame replay
live handler: allow live rewrite handler to be specified as one of the collections in pywb
by settings index_paths to '$liveweb'. When used, creates a RewriteHandler instead of WBHandler
Can also specify 'proxyhostport' to set the live rewrite to go through a proxy

fallback: allow fallback to a different handler (usually live rewrite) by specifying
'redir_fallback' with name of handler. Instead of 404, a not found response will
internally call the fallback handler to get a response
handlers: change 'redir_fallback' to 'fallback' as no redirect happens, fallback
called internally
cdxindexer: when indexing entire dir, only look at files with ext .warc.gz, .warc, .arc.gz, .arc files
and skip the rest. (Files with other ext may be specified explicitly)
replay ui: improvements to framed replay messages.
'is_live' added to live rewrite to allow for different message for live replay
vs archived replay to be used.
When using framed replay, default initial message to 'Loading...'
default index.html: list non-replay access points in default home page
update version to 0.5.0, update CHANGELIST and README