Allow import Gomfile's repo using absolute path

- Added Gomfile#itself(name = nil) method.

Gondler will create a symlink to Gomfile's directory at `.gondler/src/<name>`.
This allows golang to solve absolute import path.

- previously gondler-build can't import using absolute path, but
  relative path is not supported on `go get` command. This allows
  you to use both `go get` and gondler.
- if `name` is omitted, Gondler will try to detect it's package path
  from original $GOPATH.

For instance:

- original $GOPATH
  - src
    - example.com
      - test
        - Gomfile

The repository "example.com/test" will be available in gondler env at
".gondler/src/example.com/test" as symlink to "$GOPATH/src/example.com/test".:

- example.com
  - test
    - .gondler (gondler $GOPATH)
      - src
        - example.com
          - test -> ../../.. = example.com/test