Unit test LDLT_factor better and fix derivative bug

This pull request:
 * Adds unit tests for LDLT_factor (resolving issue #45).  The _ldlt variants of mdivide, log_determinant, trace_inv_quad_form and trace_gen_inv_quad_form are all now directly tested.
 * Fixes the gradient bug in trace_inv_quad_form_ldlt
 * Reenables the use of trace_inv_quad_form_ldlt in multi_normal
 * Uses trace_inv_quad_form_ldlt in multi_student_t_log
 * Adds gradient testing on multi_student_t_log