Add simple usage message for mirb

Fix build error on VS2012 toolchain.

A local variables define beginning of a scope block.
VS2012 unacceptable ";;" in struct definition.

Add test for Random::srand

Remove unused variable

Add test for mruby-random

Support MT instance valiables and activate mruby-random

Use C99 flexible array in pool.c

Refactoring mruby-random

Add kernel#rand, #srand ,Random#rand, #srand method

fix build error on VS2012 toolchains.

Cleanup code.