MiniSEED fixed flags - improve issue855 PR to allow datation of flags

Hello all,

Following my previous PR issue855 (https://github.com/obspy/obspy/pull/855), here is an enhanced version which allows datation of flags.

Each record is checked to know if the current flag is True at some time during the record. Record start and record end are calculated from header and blockettes 100 and 1001 and compared to flag datations given by the user.

Flag datation may be 
* a bool meaning "always True" or "always False" (same as previous PR)
* one or several "instant" datations, meaning an arbitrarily short duration
* one or several "duration", for an event that has a begin and an end. Instant and duration may be mixed inside a single flag.

Each flag is treated separately (no correlation between "begin_event", "end_event" and "event_in_progress" for example) and can be set to an "instant" value or a "duration".

Reviews and comments appreciated!

SF