Model transaction rate fetched from config file

As discussed with @jcrubino on hangouts

Currently the transaction rate is not variable. Need to fetch the config file, parse it and generate blocks as per the given height vs transaction rate.

Need a way to generate blocks according to the transaction rate so looking at enabling `SetGenerate` when number of txns are fulfilled and disabling it as soon as new block is mined.
Don't do math with floating point values. Create the Amount first, and then subtract.

The fee can be declared as:

```
var fee btcutil.Amount = 1e4 // 0.0001 BTC
```