Remove `_factor_trial_division()`
Refactor `factor()`, use `mpz_class` instead of `mpz_t`
Implement `factor_lehman_method()`
Change function signature of `gcd_ext()`
Change function signature of `mod_inverse()`
Change function signature of `i_nth_root()`
Remove trailing spaces
Fix merge conflicts

Conflicts:
	src/ntheory.cpp
	src/ntheory.h
Remove calling `iabs()` in `prime_factors()` and `prime_factor_multiplicities()`
Merge pull request #118 from certik/Refactoring

Refactoring