Bug fixes

API changes/new features:

:class:fwdpy11.regions.Sregion may now model distrubitions of effect sizes on scales other than the effect size itself. A scaling parameter allows the DFE to be functions of N, 2N, 4N, etc. PR #16
- Github issues #7, #8, and #9 resolved. All are relatively minor usability tweaks.
:func:fwdpy11.util.change_effect_size added, allowing the "s" and "h" fields of :class:fwdpy11.fwdpp_types.Mutation to be changed. commit
The attributes of :class:fwdpy11.fwdpp_types.Mutation are now read-only, addressing Issue #5 on GitHub. commit
Trait-to-fitness mapping functions for quantitative trait simulations now take the entire population, rather than just the generation. This allows us to model things like truncation selection, etc. commit

Code base updadted to work with pybind11_ 2.2.0. #19
:mod:fwdpy11.model_params has been refactored, addressing issue #4 on GitHub. The new code base is more idiomatic w.r.to Python's OO methods. commit
Many of the C++-based types can now be pickled, making model parameter objects easier to serialize. Most of the
changes are in this commit. This mostly addresses Issue #3 on GitHub
Added magic numbers to keep track of compatibility changes to serialization formats.
str changed to repr for region types commit
fwdpy11.model_params now uses try/except rather than isinstance to check that rates are float-like types.commit