[RFC] FileInfo getters for `filesize`, `hardlinks` and `blocksize`

This implements some simple getters for `FileInfo`. (continues #619)

There are still some attributes wich get accessed through the `FileInfo.stat` struct, but I would like to implement some better abstractions for them in other PR's (like #775). The goal is to make the `uv_stat_t stat` struct private.
What's the reason for `off_t` instead of `size_t`? According to [random SO post](http://stackoverflow.com/a/10634759/152142) off_t is POSIX, so I would think we should favor size_t.