Cache file data within the loaded Zip

The zip code has been refactored to ensure only a single GET request is
needed by the browser to load a document. This makes webodf slightly more
robust when loading documents as it avoids problems with the prior HEAD
request returning different file sizes, or even being unsupported by the
browser/server completely.

The current Zip implementation attempts to lazily load parts of the zip
file during decompression which is a good concept to preserve to allow
memory usage to be reduced in future. For now however, robustness of
file loading is more important.
Remove cache from browser runtime

Removing the internal cache means the XHR will fall back to the browser cache
instead. Most uses don't reload the same unmodified document multiple times
within the same session, so there should be no major problems with this
approach.
Remove now-unused runtime.isFile function
Remove now-unused runtime.getFileSize function
Merge pull request #793 from peitschie/remove-cache

Remove cache from browser runtime