Implement Special Functions

[betanalpha]

Miscellaneous special functions that have been implemented in Stan, some requiring the Boost libraries and some requiring the polygamma functions #3.

• [ ] bessel_first_kind
• [ ] bessel_second_king
• [ ] falling_factorial
• [ ] gamma_p
• [ ] gamma_q
• [ ] ibeta
• [ ] lgamma
• [ ] lmgamma
• [ ] log1m_exp
• [ ] log1m
• [ ] modified_bessel_first_kind
• [ ] modified_bessel_second_kind
• [ ] owens_t
• [ ] rising_factorial