Merge branch 'develop' of https://github.com/stan-dev/stan into develop
added raise_exception, compiles, tested by hand
added models for integration test
added tests for functions which throw exceptions
switched order of error checks, made error messages more consistent, updated unit test
cleanup - code for raise_exception stmts should be in branch issue-748, not in this one.
stan models that test error detection, error message
cleanup - wrong branch for these test models
fix multi_gp_log() signature in doc
sync multi_normal and multi_normal_cholesky

rearrange the code so that it is more evident which code parts
are shared and which are different between multi_normal and
multi_normal_cholesky distributions
use dedicated variable for Cholesky factor in tests

don't reuse Sigma variable to store its Cholesky factor
cleanup multi_gp

remove unused headers and 'using' directives,
update template parameter names
add multi_gp_cholesky distribution
Added cbind functions and tests
Added rbind functions and tests
Extra test to cbind and rbind
add multi_gp_cholesky syntax tests
cbind: functions with correct promotion of args and added agrad-rev tests
rbind: functions with correct promotion of args and added agrad-rev tests
Some basic fwd-rev tests for cbind and rbind