grand rewrite! (new features/deprecate features)

this issue will be used for changes which we would like

[ ] there are lot of changes w.r.t AdequateRecord, and lot of methods are deprecated. so, remove unnecessary code which is corresponds to old ActiveRecord
[ ] remove support for 3.x
[ ] remove support for caching https://github.com/code-mancers/invoicing/blob/a6a167d1/lib/invoicing.rb
[ ] remove patched methods like write_attributes https://github.com/code-mancers/invoicing/blob/a6a167d1/lib/invoicing/taxable.rb#L240
[x] remove deprecated sanitize_sql_hash_for_conditions
[ ] provide default models with generator rather than using acts_as. acts_as days are dead
[ ] add basic support for customizing invoice layout, which can be used by prawn to generate invoices.
[ ] make sure tests run with mysql and pg

codemancers / invoicing