Don't wait for Router and Dealer in test_changes. Fix #22.

We were waiting for rep1's router and dealer to start. This was
causing a timeout after 5s, so all the transfers had been emited
in less than 5s.
This is due to the fact that the router and the dealer only start
when all the changes have been emited.
Seek instead of appending in LC. Fix #20
Fix except clause for invalid JSON file.
Update drivers.rst

Fixed one more typo
Don't use the same session for the rules tests. Fix #36
Refactor multipass_copy
PEP8
Test should now pass
Wait for transfer started
upload_chunk and get_chunk now take metadata instead of filename

upload_chunk and get_chunk were getting the filename as first
argument. This is not wanted as a driver may request informations
from the metadata in those handlers.
Fix test_multipass_copy

The chunk size was set on the wrong driver (source instead of
target). The size of the transfer has been reduced to 100 bytes
instead of 100KB in order to have a much faster test.
Improve test_rules stability

A different was used each time, but the old files were not
deleted. Now the two reps are created before each test.
Use last version of Logbook instead of Onitu's fork
Improve error handling in LocalStorage. Fix #23

Catch errors when calling stat. Except both IOError and OSError in
all clauses.
Add Python 3.4 to tox and Travis
Add custom exceptions in the Plug
Handle exceptions in the Dealer. Fix #24
Use the new exceptions in Local Storage
Update the docs with the new exceptions
Merge branch '#24_handlers_exceptions' into develop