[wip] Create pull request
Create interface of Either class
travis.yml にlibboost-dev を追加
Fix interface of Either

* Fix using optional declaration
* Remove redundant assign operator which takes Right type
* Add swap
* Add ref-qualifier and overload to operator*
* Add non-const overload to operator->
Add implementations for Either
Merge pull request #61 from kmc-jp/eitherTravis

travis.yml にlibboost-dev を追加
Fix a bug

Write template type explicitly
Merge pull request #59 from kmc-jp/either

Eitherを作る