make instruments() method of Server accept list of currencies

Previously, the instruments() method of Server objects required a string
of comma separated currencies to be passed in. This change changes the
parameter to be an actual Python list of object.
MERGE W/ c82ada3391a4c216e66d5c5934c1aa6867a7c8ef
MERGE W/ 9a626ca5
introduce EMA strategy
MERGE W/ c96a208621f2b96a56b4d1d620e9891183e71369
MERGE W/ c96a2086
MERGE W/ f21e8253
enhance EMA calculation
continue work