Add starting and current priority to getrawmempool verbose mode. 

This PR adds functionality for filling in the `StartingPriority` and `CurrentPriority` fields for the `getrawmempool` RPC command in verbose mode. 
This fixes issue #178.

The interface for computing a mempool tx's priority has been exposed as two new public functions for the `TxDesc` struct: `CurrentPriority` and `StartingPriority`. 
A new field: `startingPriority`, has also been added to `TxDesc` for the purpose of computing and then caching a tx's starting priority. 

Portions of `mining.go` have been refactored to use the newly created function. 

I've been testing the changes to `mining.go` over the past few days on the testnet to ensure I haven't introduced any breakages. A minor bug was found and subsequently fixed, but everything seems to be running smoothly since then. Any additional testing would be appreciated.