Add handler for `GET /repository/repo/branch` (fix #123)

Tests were named like that (`TestGetBranch`, `TestGetBranchWhenRepoNotSupplied`, `TestGetBranchWhenRepoNonExistent` and `TestGetBranchWhenCommandFails`) to gain on such calls:

```
$ cd api/
$ go test -gocheck.f TestGetBranch ./...
OK: 4 passed
PASS
ok      github.com/tsuru/gandalf/api    0.167s
```

```
$ cd api/
$ go test -gocheck.f TestGetBranchWhen ./...
OK: 3 passed
PASS
ok      github.com/tsuru/gandalf/api    0.158s
```

```
$ cd api/
$ go test -gocheck.f TestGetBranchWhenRepoNo ./...
OK: 2 passed
PASS
ok      github.com/tsuru/gandalf/api    0.126s
```

```
$ cd api/
$ go test -gocheck.f TestGetBranchWhenRepoNon ./...
OK: 1 passed
PASS
ok      github.com/tsuru/gandalf/api    0.129s
```