Replace SHA256 implementation

Found on http://www.bichlmeier.info/sha256.js
License: 3-clause BSD license

The code is less adapted from embedded system with redards to size, but
will hopefully work better.

Sizes of sha256.o:
```
Before After Board
10904  12492 avsextrem
 8412  11056 chronos
53896  30956 mbed_lpc1768
 9240  11616 msb-430h
 9240  11616 msb-430
10904  12492 msba2
 2632   2684 native
10904  12492 pttu
  n/a   3416 redbee-econotag
 9240  11616 telosb
 9240  11616 wsn430-v1_3b
 9240  11616 wsn430-v1_4
```

Only tested for native.
Previously test_sha256 did not build for redbee-econotag.