beatracker: Add a gen_server to track time/beats

[oubiwann]

Blocks:

• beatracker: Provide the ability to start and stop music playback #23

Part of:

• Add beatracker #19

[oubiwann]

lfe> (undermidi.beatracker:ping)
PONG
lfe> (undermidi.beatracker:state)
#M(controlling-process undermidi.beatracker name beatracker
   table-desc "Beatracker table" table-name beatracker)
lfe> (undermidi.beatracker:table-info)
#M(compressed false controlling-process undermidi.beatracker
   decentralized_counters false description "Beatracker table"
   heir none id #Ref<0.2243269296.1312686081.123284> keypos 1
   memory 311 name beatracker named_table true node nonode@nohost
   owner #Pid<0.653.0> protection public read_concurrency false
   size 0 type bag write_concurrency false)