Efficient implementation of multiply_lower_tri_self_transpose.hpp
Modified test file to include symmetry checks on large matrices
updated test
Removed old method from unit/math/matrix test, changed the large L matrix generator function, added the test to agrad/rev/matrix test, cleaned up commented code
small fix for beta distribution error messages
Added neg_binomial_2_cdf + tests
Added neg_binomial_2_cdf_log + tests
Added neg_binomial_2_ccdf_log + tests
added neg_binomial_2 cdfs to functions signature and syntax tests
added neg_binomial_2 cdfs to manual
Fix misc typos in manual and RELEASE notes

- cbind and rbind to append_col and append_row
- reference to ode_solve changed to integrate_ode
- fix font rendering problems in boolean operators
added emacs temporary files to gitignore
Improvements and fixes to neg_binomial_2 cdfs
More tests for neg_binomial_2 cdfs
missing using statement
removing example-models repository
renaming var_context_combined.hpp to chained_var_context.hpp