Prevent spectral density plots being triggered in identification

If options_.SpectralDensity.trigger is specified, identification would
otherwise plot graphs in every MC step
Make graph titles more expressive
Make field names consistent with different analysis steps and manual

still related to #316 (https://github.com/DynareTeam/dynare/pull/316)
Store MCMC information recorded in record in oo_

Closes issue #315 (https://github.com/DynareTeam/dynare/issues/315)
Add endogenous priors

Add option and code for endogenous priors according to
Christiano/Trabandt/Walentin 2011, JEDC. Still needs to be integrated to
manual and pre-processor.
Add warning for variance decomposition at order=2

At order=2, the variance decomposition is still based on order=1. This
commit introduces a warning. See issue #278
(https://github.com/DynareTeam/dynare/issues/278). This information that
the variance decomposition is only triggered at periods=0 has been added
to the manual.