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` doesn't allow `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` parameter is omitted, Gondler will try to detect its package path
  from original $GOPATH. If `name` parameter is given, Gondler will use
  that value for package path, instead of detected path.

  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".:

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