absolutize the path of target file

I fixed the problem in #1.

After this fix, sample of command line log is as following:

```
% pwd
/Users/issm/sandbox/go/hoi
% ./hoi hoi.go
http://192.168.1.154:8081/cn2yll4jrrs5s8c9fslwtucze1rx672e/hoi.go
% tree ~/.hoi
/Users/issm/.hoi
├── conf.json
└── temp_public
    └── cn2yll4jrrs5s8c9fslwtucze1rx672e
        └── hoi.go -> /Users/issm/sandbox/go/hoi/hoi.go

2 directories, 2 files

% curl -I http://192.168.1.154:8081/cn2yll4jrrs5s8c9fslwtucze1rx672e/hoi.go
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 1816
Content-Type: text/plain; charset=utf-8
Last-Modified: Sat, 19 Jul 2014 07:32:04 GMT
Date: Sat, 19 Jul 2014 07:58:19 GMT
```