Conserve Launchpad's bug status values

Bug Repo Syncer has internally only 4 status values, while Launchpad has 14
status values. Several of Launchpad's status values are considered equivalent
to one internal status value. For example: "New", "Incomplete" -> "new".

Internal Status Values:
    "new", "confirmed", "in_progress", "closed"

Launchpad's Status Values:
    "New", "Incomplete", "Opinion", "Invalid", "Won't Fix", "Expired",
    "Confirmed", "Triaged", "In Progress", "Fix Committed", "Fix Released",
    "Incomplete (with response)", "Incomplete (without response)", "Unknown"

Currently when bugs are updated in Launchpad, the status values are adapted to
the much simpler internal model. A bug with status "Incomplete" gets a new
status of "New". This leads to loss of information on information on Launchpad.

'''Therefore:'''
Don't overwrite bug status values on Launchpad, when they are compatible with
the internal status values.

The architecture of Launchpadlib makes this possible without further slowdown
(I think). Every bug on Launchpad is downloaded anyway before it can be
updated. Therefor the program can look at the bug status before it is changed.
It would turn Launchpadlib's slowness into its strength.

See also:

bug #959770