Add schema dumper

[danini-the-panini]

Aims to resolve #25

Adds a schema dumper to add constraints to db/schema.rb, so that this gem can be used without having to switch to structure.sql

TODO:

• [ ] Check
• [ ] Foreign key
• [ ] Unique
• [ ] Exclusion
• [ ] Presence
• [ ] Inclusion
• [ ] Match
• [x] Numericality
• [ ] Null
• [ ] Presence
• [ ] Length