add conda-forge channel to installation docs

Merge pull request lmfit#353 from lmfit/prep094rc2

Prep094rc2

Merge pull request lmfit#343 from lmfit/prep0.9.4

Prepare 0.9.4

more doc tweaks

Merge pull request lmfit#315 from lmfit/issue314

refactor Model.set_param_hint()

Merge pull request lmfit#249 from andyfaff/emcee

ENH: adding an emcee sampler to Minimizer

Merge pull request lmfit#270 from lmfit/model_conf_interval

Model conf_interval

put setattr() in try/except

Merge pull request lmfit#257 from lmfit/exprmodel_noprefix

add warning and doc for prefix in ExpressionModel

Merge pull request lmfit#255 from danielballan/fix-ipyfitter

Fix ipyfitter