Change GMPs default PRNG to Mersenne Twister

Since we don't allow users to seed the RNG, there is no BC issue with changing the generator.
Add gmp_random_bits(bits) and gmp_random_range(min, max)
Add test files
Mersenne Twister was added in GMP 4.2
Make sure min < max
Address issues raised by @nikic

 - Thread safety on rand init function.
 - Ret false on validation failure
 - Add _dep of temp_a to temp_b
 - Special case int sized min values
 - More tests!
Merge branch 'PHP-5.6' of https://github.com/php/php-src into gmp-random
Upated NEWS
Merge branch 'pull-request/839' into PHP-5.6

* pull-request/839:
  Upated NEWS
  Address issues raised by @nikic
  Make sure min < max
  Mersenne Twister was added in GMP 4.2
  Add test files
  Add gmp_random_bits(bits) and gmp_random_range(min, max)
  Change GMPs default PRNG to Mersenne Twister
update NEWS and UPGRADING