[Jira] Fix #43. Incremental support now works with multiple trackers

Add the logic needed by the function get_last_modification_date of
the Jira backend to support multiple trackers and multiple "last
dates" in the same database.
[Jira] Fix #52. 'changes' table stores now the account name of the
users instead of the real name (which is not unique).

Bicho's JIRA backend was extracting incorrectly the information
from changes. It was storing the real name instead of the user
unique id.
[logtable] Fix error processing no one email entries
[logtable] Incremental support for multiple trackers
Debian package files for version 0.14.2