session/new source for unite

Added a session/new source so that users can save sessions using only unite without needing to call :UniteSessionSave or :UniteSessionLoad. This source also protects against overriding an existing session. Only new sessions will be created, so it's safer to use this source than :UniteSessionSave. 

This update is fully backwards compatible. 

Thank you for considering adding this feature. 

Jason

PS 日本語も分かります。