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* [Writer] Remove a lot of memory copy.
* [Writer] Add xapian indexing directly in libzim.
* [Writer] Better API.
* [Writer] Use multi-threading to write clusters.
* [Writer] Ensure mimetype of articles article is not null.
* Extend test timeout for cluster's test.
* Less memory copy for cluster's test.
* Allow skipping test using a lot memory using env variable
`SKIP_BIG_MEMORY_TEST=1`
* Explicitly use the icu namespace to allow using of packaged icu lib.
* Use a temporary file name as long as the ZIM writting process is
not finished (163)
* [Travis] Do no compile using gcc-5 (but the default trusty's one 4.8)