Feature/fix misaligned access

Using static irep binary (3492be4) fails on alignment-strict processors.
This patch adds padding bytes after and each header and block (iseq/pool/syms) to align to 4-byte boundary.
Note that dumped mrb format has changed.
(RITE_BINARY_FORMAT_VER "0002" => "0003")

I have tested on following machines:
- Intel x86_64 (gcc 4.8.3)
- Altera Nios II -- alignment-strict processor (nios2-elf-gcc 4.7.3)