Allow import Gomfile's repo using absolute path

* Added :path option for Gomfile#gom.
  - This adds symlink to specified path instead of `go get` and
    installing.
  - Relative path from Gomfile directory is supported.

* 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