CSV reading: continue without explicit dialect upon guess failure

Rationale: the default settings of Python's csv module allow to read several
CSV sub-formats without having to specify a dialect. With this change we allow
to read them even when the CSV sniffer is unable to determine the actual
dialect in use.
Merge pull request #70 from mlamby/feature-account-list

Optionally read account definitions from a ledger file
Merge pull request #71 from zacchiro/master

CSV reading: continue without explicit dialect upon guess failure
Merge branch 'master' into feature-retry