Fix for home directory not found on Windows

After cross-compiling `gof3r` for `windows/amd64` and running the binary
I got the following error,

home directory not found for current user
![screenshot](https://cloud.githubusercontent.com/assets/6806679/5286223/bc280b00-7b22-11e4-86f6-d0b1fd9090da.png)

The fix for this error is to use the third-party package `go-homedir`, which
detects "the user's home directory without the use of cgo, so the library
can be used in cross-compilation environments."