Improve code for Dev OMNI calculations

[msgilligan]

The current code for Dev OMNI (formerly Dev MSC) calculations (omnicore.cpp: calculate_and_update_devmsc()) should be improved in one or more of the following ways:

1. Refactor for clarity and improve comments -- see PR #1246 which is a first step
2. Total Dev OMNI value is hard-coded for Omni main net and is not verified anywhere (that I know of) against the actual crowdsale OMNI totals
3. The calculations use floating-point (double) and may have significant rounding errors or be platform-dependent
4. More unit-test and functional tests
5. The specification itself should probably be revised and/or clarified -- token issuance should use an algorithm similar to the Bitcoin issuance (i.e. using block height rather than timestamps and specifying per-block issuance directly in satoshis/willetts)