Add some macros for `reader::parser`
Add some macros for `reader::parser`

try_with_pos: try!-like macro, which may return ok value or Error.
try_read_le_u32: try to read u32 as little endian.
                 Change `$pos` if succeeded, or return Error if failed.
try_read_exact: try to read exact size of `u8`s.
                This is alternative to `std::io::Read::read_exact()`
                (because it is not stable in Rust-1.5 yet).

l1048576/fbx_direct