Add Keyset package

[Yeti-or]

class Keyset to work with block.i18n directories

keyset contains LangKeys for each lang

keset could be parsed/serialized in different formats right now:

• enb – format that SERP is using right now
• taburet - our bright future format

TODO: https://github.com/bem/bem-sdk/issues/349

[jabzzy]

только доку давай следом, а не сейчас