Replace unicode() with u() in theme.py

Fixes #271
Add ambiwidth problems information to troubleshooting section
Do not throw away python stack trace

Errors when loading powerline fails are almost never giving any useful
information for debugging.
Fix minimal python versions

s/3\.2/3.3
Use exit_fail hl group for last_status segment

Fixes #270
Remove obsolete highlight_group keys from configuration

They are no no-op for function segments, so don’t confuse users
Also use hl_iter in exception message

Ref #270
Improve documentation
Add _external_ip for use in weather segment
Add support for python-3.2
Add support for python-2.6
Use mode(1)

Otherwise all that fancy double-symbol modes are just not visible.
Start adding tests

Tests are temporary disabled on all branches except tests (does not really work
though, needs travis.yml in all branches).
Add tests.lib module with `replace_*()` with statement functions

Note: there is new problem: now multiprocessing() globals got assigned None
values. It is likely somehow related to extended `sys.modules` utilization
Omit using sys.modules if possible

Does not work though: still TypeError due to some global in multiproccessing
that got None
Add support for with: statement to tests.vim

Also made everything use new vim_module._with where possible
Make tests support python2.6

Old unittest is not working for unknown reason, using unittest2 instead
Do not restrict branches to `tests` branch

In any case it was not working
Always return a string in email_imap_alert segment

Fixes #279
Update credits and PR guidelines