New market API uses doubles

[theoreticalbts]

The new market API implemented as part of #503 uses doubles. Use of doubles in any financial system is a worst practice, and this situation should be corrected. If #246 is finally implemented we should isolate the double's into their own API.

An alternative version is to refactor the JSON code to take a parse_float function pointer, like Python's JSON parser, and then have the floats parsed as some kind of rational or string class that doesn't lose information.

[vikramrajkumar]

This issue was moved to bitshares/bitshares-core#144