Implementing storehaus with Postgres backend

[ponkin]

Hi everybody. Here is storehouse implementation for postgres. Need your review and comments.

[ponkin]

@johnynek Sorry for long delay. I added MergeablePostgresStore and tests for it. Also I rewrote all with Injections. I was trying to keep overall style. please review)

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:x: Alexey Ponkin
:x: ponkin

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