Simple population dynamics system based on the ideas described in #3.

- Add the `ipdt popdyn --generations 10` command
- Add the popdyn.py module that run a small population dynamics model,
  where the geometric growth rate of the proportion of a strategy in the
  population is computed from its payoff weighted by its abundance.
- (DRAFT) HTML visualization of popdyn experiments with a D3.js script (use --html)
No Normalization if max==min. Fixes #8.
added a small mutation process
Added a command to control mutation level.
Style adjustment for popdyn export
Export for popdyn
Documented popdyn
typo fixing in readme
added popdyn in example output
Merge pull request #9 from geeklhem/popdyn

Added a simple population dynamics model.