Re-factored default_beta_ladder outside PTSampler object, and added optional arguments for specifying Tmax instead of ntemps.
Added flatchain property to ptsampler.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	emcee/ptsampler.py
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/dfm/emcee

Conflicts:
	emcee/ptsampler.py
Fixed bug related to recent upstream merge in ptsampler.py
Merge remote-tracking branch 'upstream/master'
Changed to p(z) ~ 1/z choice of scaling parameter in stretch move.
Merge pull request #111 from farr/master

PTSampler Temp Ladder Changes Merged with 2.0.0