Implement dynamic descent token exchange rate algorithm

[yuanfeiz]

Algorithm:
bodhiToken = ether * tokenExchangeRate()
tokenExchangeRate = initialExchangeRate - (block.number - fundingStartBlock) / 1000 * 0.1

Note: use safeMath.

Tests:

1. before fundingStartBlock
2. at fundingStartBlock
3. after fundingStartBlock, before fundingEndBlock
4. at fundingEndBlock
5. after fundingEndBlock
6. at the cliff