Add ZIP reports for financial ledgers

New module: given that too many entries can make the general ledger/partners ledger reports fail due to lack of resources (SELECTs with too many move line ids, wkhtmltopdf lacking free memory space to generate the file, etc.), they can now be printed in "split" PDF files, all regrouped in a ZIP archive.

Rules for splitting files:
- partners ledger: one subfolder per account, max. 20000 ledger lines per account (a partner is added all at once). The files will be named according to the first and last partners in it.
- general ledger: one subfolder per account, 20000 ledger lines per file, opening balance included. Numbered 1-n at the end.
For both reports, a "balance" file is also created at the root of the archive, since the Webkit reports depended on the lines present in it to compute it.

What's still needed:
- unit tests,
- probably create a new module 'report_zip', akin to 'report_webkit' and 'report_xls', allowing to create new reports more generically,
- any improvements to the report layout/file naming convention.