- Modified SQLite's representation of "microseconds" to

- Modified SQLite's representation of "microseconds" to
match the output of str(somedatetime), i.e. in that the
microseconds are represented as fractional seconds in
string format.  [ticket:1090]
- implemented a __legacy_microseconds__ flag on DateTimeMixin which can
be used per-class or per-type instances to get the old behavior, for
compatibility with existing SQLite databases encoded by a previous
version of SQLAlchemy.
- will implement the reverse legacy behavior in 0.4.