hbstabilizer: conversions added

The original proposal for the HBD stabilizer included the following:

If the price is below LOW, use the HBD to convert into HIVE (takes 3.5 days). Upon completion of the conversion, use the HIVE to buy HBD and send that back to the DHF

This was not originally implemented, and has now been added, with a small modification. Rather than committing to use HIVE to buy HBD, the decision will be made at the time the conversion completes. If the market price of HBD at that time is above $1, the HIVE will be sent to the DAO instead (which performs an automatic conversion assuming HBD=$1).

Because there is some risk of conversion not resulting in a profit (due to market fluctuations during the 3.5 day period), LOW has been set to 0.97. This provides some safety margin.

100% beneficiary to @hbdstabilizer