grand rewrite! (new features/deprecate features)

[iffyuva]

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
• 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

[iffyuva]

/cc @ept