caching the zip manifests Fixes #154

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extra : rebase_source : 0f32792e01b6a1b746a1e07ffa4d7a85eeda1595
Must add zipinfo as a property

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extra : rebase_source : 2ae5fa38bd5c9d37b44a010879f2b3a4867c0ee4
Summarize zip_manifest cache changes.

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extra : rebase_source : b4dc90d0244a83083a299600eccce50446ad5846
Remove excess whitespace
Subclass dict rather that wrap and proxy.
Expose a method describing what it does, and alias that to 'call'.
Implement 'build_manifest' as a classmethod. Rename to 'build' because Manifests is already in the classname.
Rewrite construct/append loop with simple iterator.
Use direct voice
Remove documentation which seems irrelevant to this method.
Use a more appropriate name and invoke .load directly.
Stat is never reused
Use a namedtuple to avoid numeric indexes
Use __import__() to avoid leaking imported names to the executed script.
Merge with master
Create zip_manifests as a class attribute rather than a global

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extra : amend_source : 7754d23956c4157235dca41e90f05c29691078ee
Split MemoizedZipManifests from ZipManifests. Ref #154.
Make memoized zip manifests opt-in using the PKG_RESOURCES_CACHE_ZIP_MANIFESTS environment variable. Ref #154.
Merge with master

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extra : amend_source : 272b2718bac5934550a589d08f584120ba78086a