Implement write access

[molnarg]

It would be nice if entries wouldn't be read only.

[molnarg]

If the entry size does not change, it's pretty straighforward to implement it right? For my usecase, it would be enough.

[molnarg]

I wrote a quick and dirty monkey patch to support overwriting data: https://gist.github.com/molnarg/7371947

I don't propose it as pull request since it's a partial solution and doesn't have any testcases. But maybe someone could use it in the future.