proper decoding of nanoseconds in TimeUUID

Currently, the TimeUUID implementation does not properly parse nanoseconds.
The [TimeUUID specification section 4.1.4](http://www.ietf.org/rfc/rfc4122.txt) states the timestamp is a count of 100-nanosecond intervals.

The existing code retrieved the bundle count, not the actual nanosecond count (simply multiply by 100).

The issue was discovered while inserting sequential rows in Cassandra at ~1ms intervals, yet the parsed times differed by only a few nanoseconds. The associated change (nsec * 100) gave us values that lie within the margin of timing error.

Tests in `uuid_test.go` have been changed to test the nanosecond resolution as well.

Example on playground:
http://play.golang.org/p/FitgDTJ2Dd