Misc fixes

[skirpichev]

• Add warning about possible crash of the interpreter, closes Overflows causing gmp: overflow in mpz type Aborted instead of Python OverflowError #280
• Add regression test, closes is_divisible() gives wrong answer for integers on 64-bit boundary #339
• Add regression test, closes is_prime() returns True for "negative primes" #312
• Special case for k==0 in lucasu/v*, closes About the case where k=0 is input to lucasu_mod and lucasv_mod #445
• Convert line endings in master to unix format
• Add coverage tests for powmod_sec(), closes mpz_powm_sec is not exist #324