Added the `RequestLog` Model to the possible exports

Added `gzip`, `tgz (tar.gz)` and `tbz (tar.bz2)` to compression methods for exporting
Added mime-types for the contents to the response.

Changed rendering in an abstraction method `_render_json`,
to implement further render methods for `protobuf` or other similar structured datatypes.
Merge branch 'develop' into export-requestlog
Missed the changes in the template to use new exports.
Merged the changes from the `adhocracy.hhu_theme`.
Merge branch 'develop' into export-requestlog
Removed the "csv" option from the formencode, the code responsible to handle this option is thrown out, due to nested dict structure of the export data, that the "csv" format not easily could represent.
Merge remote branch 'skoenen/export-requestlog' into develop
Revert "print coverage report to terminal"

This commit leads to an insane amount of output when running the tests interactively and makes it hard to see whether the tests failed - one has to scroll up 10 pages to see the test results.

This reverts commit 87e77bc7af2b4f76a2f77b5145bfe7634e4c0c9a.
Simplify importexport

See hhucn/adhocracy.hhu_theme#350 for some details
Merge branch 'develop' of https://github.com/liqd/adhocracy into develop