fix commit 810bc5fe005ce1d (aka 'Finally fixing plots.' ;)

In 810bc5fe005ce1d, the auto-logic for sleak min-max plots of traces
with too many samples was changed, resulting in every plot with no
method explicitly specified being plotted in "full" mode.
Update README.md

add gitter badge linking to obspy/obspy chat room
add starting sequence number support for MiniSEED files
add sequence_number as optional argument of writeMSEED