refactor target_unit to base_unit
refactor price_unit to quote_unit
Remove length restriction on market id (ref #59 #76 #190)

This change also make base_unit/quote_unit configurable in
markets.yml, for maximum flexibility and compatibility.

Update markets.yml in production environment required.
update travis markets.yml
Merge pull request #252 from peatio/variable-length-market-id

Variable length market