* proxy: add option to set default timestamp for proxy mode, fixes#452
- set via flag --proxy-default-timestamp or config 'proxy_options.default_timestamp'
- can be iso date or all-digit timestamp
- overridable via accept-datetime header
* docs: update docs for proxy timestamp
- add docs on memento support in proxy mode
* update-version: script can update version only, commit with 'update-version.sh commit'
* indexer post append: remove 'WB_wombat_' from POST query, could have been added in previous versions of pywb!
* versioning: new MAJ.MIN.DATE versioning
move version to version.py for easier updates
add update-version.sh for autoupdating version in version.py, pushing new tag with current version