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

[scorphus]

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

[andrewsmedina]

@scorphus you forgot to map a url to your handler.

[andrewsmedina]

@scorphus thank you!