better handling of preparing "path" for GET requests

can't just use:

``` r
url <- "http://string-db.org/api"
```

as httr:::handle_url in GET cuts it off. so `api` even though it's the only access point so far can't be baked into url.

for reference the full request url:

``` r
"http://string-db.org/api/tsv/resolve?identifier=ADD&species=9606"
```
