discoal v0.1.1

This is the first release of discoal using the "releases" feature of GitHub. Kevin Thornton suggested that I use this feature to track changes and I agree it's an excellent idea. v0.1.1 fixes the way I had been dealing with the origination of alleles used for conditional trajectories, both neutral and beneficial. In particular I had not been accounting for changes in the mutation rate of these alleles that we condition upon (see Slatkin (2001) Simulating genealogies of selected alleles in a population of variable size). Aaron Stern noted this potential problem and suggested a fix might be to use rejection sampling of trajectories. This is now implemented in v0.1.1. All said the update has very minor effects on summaries of variation from the vast majority of demographic models, so past work done with v0.1 is likely unaffected by my previous implementation.