docs: use parallel builds to speedup docs generation

Using parallel builds to increase doc regeneration as well as speedup
Travis builds.

With hot filesystem speed are as follows depending on the -j value:

Processes	Duration
1	7.75s
3	5.2s
5	4.21s
8	3.88s
10	3.96s
15	3.38s
20	3.48s

Cold (12 runs)
1	20.27 (17.82-22.71)
15	17.91 (14.41-21.42)

Not as big a saving as I'd hoped for a cold start but good for doc
development.
Travis: disable unsupported DB backends and Django versions

They're known to fail and we can reenable them when support is brought
back into core.  For now they're simply making Travis take longer to
report issues.
Travis: install lxml without compile optimisations

Use CFLAGS="-O0" as recommended by
http://lxml.de/installation.html#installation to speedup installation
times in Travis builds.

This is applied to all installations here, should maybe be limited to
lxml.

The next step wpuld be to create precompiled wheel packages to speedup
setup.

Without this change we take on average 2,5min to setup the packages for
testing. After this we shave off 1,3min i.e. setup takes on average 57s
gitignore: .DS_Store
Cleanup: removed some permissions left behind

These should have been gone in 6b6b5532.
pep8: --help options was broken in no param case
pep8: document exclude patterns
pep8: adjust line length to beyond current max

Catch line lengths based on the Pootle Styleguide
http://docs.translatehouse.org/projects/translate-toolkit/en/latest/developers/styleguide.html#maximum-line-length
Tests: fixed docstring
Tests: removed dead code
Cleanup: removed dead code
Admin reports: Render precise values for totals in detailed reports
Record submissions for comments added from update_store command

Fix #3376
Reports: replace date with datetime in Paidtasks

- Change `date` PaidTask field to `datetime`;
- List paid tasks in Detailed Statistics.
- Show dates in ‘Registered tasks’ table in Admin/Reports and Statistics;
Reports: fix date interval when time was changed to (or from) DST
Reports: allow to add corrections as paid tasks

- Change amount field from positive integer to float;
- Add new paid task type;
- Use enum type instead of unnamed constants;
Reports: Clear effective_from field after user rates updating
Skip rqworker cache missed errors in DEBUG mode
Travis: Broadcast build failures on IRC