filesize limit of 4MB

Trying to work with files bigger than 4MB results in misleading errors.

Changing the size in the tests.py to (1024**2 \* 5) will result in:

```
File "python-librsync/librsync/__init__.py", line 126, in _execute
  block = f.read(RS_JOB_BLOCKSIZE)
File "/usr/lib64/python2.7/tempfile.py", line 581, in read
  return self._file.read(*args)
```

  IOError: File not open for reading
