have repo tilde expand paths

[whyrusleeping]

As I work through examples, I feel like it would be much easier for the fsrepo to tilde expand paths given to it. This makes it much easier for users to write the following code:

r := fsrepo.At("~/.go-ipfs")

instead of:

home := os.GetEnv("HOME")
if len(home) == 0 {
    panic("WHAT IS HAPPENING?")
}
r := fsrepo.At(home + "/.go-ipfs")

[jbenet]

I'm fine with this. i dont know all the places its used and whether that violates any assumptions. but tests pass so i guess that's fine.

[jbenet]

changes lgtm

[whyrusleeping]

I think that anything user facing should accept as wide an array of inputs as possible