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

Merge pull request lmfit#253 from lmfit/rc_090

update docs, near ready for for 0.9.0rc

Merge pull request lmfit#195 from newville/master

add dump()/load() to Parameters class

Merge pull request lmfit#188 from newville/master

doc: add start of FAQ, and better links to support