Remove cache from browser runtime

This is an alternate approach to the issue that PR #712 is attempting to solve. There are two parts to this patch:

#  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.

# Review questions
* Should the now-unused ``isFile`` & ``getFileSize`` functions be removed from the Runtime interface? Using these in a browser environment may cause the complete file to be downloaded multiple times if the server doesn't support the HEAD method, or under some CORS scenarios
* Should other unused file methods be discarded? E.g., ``deleteFile``?