Fine grained invest()

At present `invest()` buys in bulk. That is, it gets the price from `getInvestmentPrice()` and purchases `N` market token gwei at this price. This gives an advantage to bulk orders from early adopters since they can purchase up a lot on the cheap (credit to Rei Chiang for this attack).

A more robust scheme would be to have fine grained purchase. Let's say that only `M` market token gwei can be purchased at a given price. (If `M = 1e9`, then one market token max can be purchased at the set price). Then to purchase `N` total, you would do a for loop:

```
remaining = N
while remaining > 0:
price = getInvestmentPrice()
purchase min(remaining, M) gwei of market token at price
remainder -= min(remaining, M)
```