Only read first 512 bytes

Since only the first 512 bytes are considered, only read the first 512 bytes instead of the entire file.

This improves performance noticeably for files in the hundreds of megabytes:

```
> ls -al /tmp/huge.zip
-rw-r--r--  1 kevin  wheel  237545472 May 24 12:24 /tmp/huge.zip

> git co master 
> time bin/isbinaryfile /tmp/huge.zip
real	0m0.250s
user	0m0.067s
sys		0m0.183s

> git co read-first-512-only
> time bin/isbinaryfile /tmp/huge.zip
real	0m0.101s
user	0m0.065s
sys		0m0.035s