Merge pull request #56 from abernardes/master

Extend sequel adapter for all supported RDBMS

closes #37
Typo in @modeL_opts
Merge pull request #57 from adz/typo_in_modeL_opts

Typo in @modeL_opts
Simplify and extend mapper definition DSL

  * ditch nested blocks in favor of `define(:rel_name)` syntax
  * add ability to define mapping attributes via `attribute` interface
  * add ability to exclude attributes via `exclude` interface

refs #52
Add guard and bump rspec
Refactor mapper and model builders

  * Move some responsibilities to `ModelBuilder`
  * Remove :map options from `model` interface (it's reduntant)
  * Set model_class in the initializer if it's available

refs #53
[ci-skip] update readme
MapperRegistry => ReaderRegistry
Improve Header interface

  * Make it more hash-like
  * Add `Attribute.coerce`
  * Fetch attributes via method_missing for convenience

refs #53
Expand Header interface with the support for collections

  * Remove method_missing trick since it could be in conflict with
    attribute names and header instance methods
  * Make `ModelBuiler#call` accept a header rather than attribute names
  * Add support for defining multiple group mappings

Refs #52 #53
Prepend adapters as they are registered

Appending adapaters as they are registered doesn't allow a programmer
to override a previously-registered adapter, meaning that a misbehaving
library can force everyone to use a particular adapter for a given
URI scheme.

This simply prepends adapters as they are registered, implementing
a "last-loaded wins" strategy for picking which adapter to use for
a given URI scheme.
Added spec to verify last-added wins adapter registration system
Add RA.wrap

refs #58
Add support for wrap operation

closes #58
Extend Header to support embedded hash (refs #58)
Add support for renaming attributes (closes #61)
Bump sequel
Update README
Change relation dsl to use `register(:rel_name)`

  * This allows to easily register a relation without the need for block
  * Simpler implementation
  * Probably easier to understand
Update gemspec and remove runtime sequel dependency