Add unsigned support for numeric data types

[daichirata]

Hi,

A data type of unsigned is supported from Rails 5, so this pull request supports it.

Example:

# Schema Info
#
# Table name: users
#
#  id      :integer          not null
#  integer :integer          unsigned, not null
#  bigint  :bigint           unsigned, not null
#  float   :float            unsigned, not null
#  decimal :decimal(10, 2)   unsigned, not null
#

[coveralls]

Changes Unknown when pulling 8f323de on daichirata:feature/unsigned into * on ctran:develop*.

[ctran]

Thanks for the PR. I had to rebase against develop and added some minor changes to make rubocop happy at #393. I'll close this one out.