Clarify prefilter option in estimation
Document treatment of missing observations
Closes #286
Provide more information on the loglinear option and its data treatment
Remove constant and noconstant options from manual for estimation command

Those options are set internally depending on whether the steady state is zero or not. See http://www.dynare.org/pipermail/dev/2014-January/003655.html
Filter out inconsistent specification of prefiltering when observation equation is not mean zero
Issue an error message if loglinear option is equal to 1 in non linear likelihood routine.
Merge branch 'johannes-documentation'
Test if the data are positive before applying the log.
Rewrote the error message issued when the prefilter option in used while the steady state of the observed variables is non zero (introduced in commit d82252e805c007a99de353e49d1a3fa2f961d748).