Add endpoint config and enable to run test code

[joker1007]

Changes

• Add endpoint config in order to use dynamodb-local.
• Add primary_key, sort_key config because createTable needs them
• Use dynamodb-local when developer run test.
• Fix createTable because current version doesn't set necessary parameters.
• Change gem task dependency from build to assemble
  • because that If developer doesn't have dynamodb-local, he cannot build gem, avoid it.

[sakama]

Thank you for your PR.
I have 1 question. primary_key and primary_key_type are defined as non-optional string.
I think they're unnecessary when user doesn't use auto_create_table. So both of them can be defined as Optional.

[joker1007]

@sakama
You are right. I fixed it.
Could you review again?

[sakama]

Thanks! I merged.
Sorry I took a long time.

[sakama]

I released v0.1.0 so you can download it from rubygems finally.
Cc: @satoshihirose @YutaroSoejima