simpler initialisers
added `porcentaje` to ConceptoRetencionFuente

Also updated seeds with new ConceptoRetencionFuente’s percentages
renamed ConceptoRetencionFuente's DB table

from `concepto_retencion_fuentes` to `conceptos_retencion_fuente`
added bootstrap 3 date picker and Bandango.DatePicker
updated template and added date picker initializer. This closes #9
CompraSerializer and VentaSerializer now inherit from OrderableSerializer
moved inflectors to it's own file

And added inflector for *retencion*
minor tweaks

- avoiding N+1 queries on Api::ComprasController and
Api::VentasController
- bugfixes
minor changes on implementations

- *includeForm* option is now defined at `OrderableFormView` to avoid
code duplication
- `OrderItemsTableView` now inherits from `TableView`
- `TableView` includes a `includeForm` binding to it’s parent view
added `numeroAutorizacion` to `Retencion`

And some fixes on relationships
added RetencionesBaseMixin and RetencionBaseMixin
added support for ice
changed templates
extracted and implemented Retencion form
changed Retencion fields

added: `numero_serie_establecimiento`, `numero_serie_punto_emision`,
`numero_serie_secuencial`
removed: `numero_retencion`
extracted to a new method so that others can override it
changed `persistedRetenciones` implementation
template tweaks
updated Bandango.RetencionFormView
added `#to_s` to ConceptoRetencionFuente and simplified templates