Add the new LVC_EARLY_WARNING event class (closes #47), and include a sample event.
Raise the default exposure times to 90s from 60s, to match the current survey. Swift and short Fermi GRBs (which already did 9x60s) now do 6x90s, everything else does 4x90s.
Add new classmethods to Event classes (closes #41).
Event
classes (closes #41).