Working ModuleBuilder

* global configuration
* support existing API
* add Virtus configuration methods
* add Virtus.module
* very rough ModuleBuilder (not working)
* specs for configuration and virtue methods
* specs pass
* use define_singleton_method to reduce noise in
  add_included_hook
* pass unencapsulated coercer to Attribute.build
  as :configured_coercer
* handle coercer configuration if configured
  and passed in as an option
Add doc to lib/virtus
Move global configuration specs
Remove Configuration#initialize spec

Initialization is covered in call_spec
Document merge_options
Deprecate Attribute.coerce

Remove `coerce` from Attribute options and add a
coerce method with a deprecation message.
Attribute.coerce just proxies the value to
Virtus.coerce.

Update Attribute.writer_class to use a default
coerce setting and update specs to reflect the
coerce removal from options.
Remove usage of ||= in Configuration
Remove unused block capture in Configuration
Document Configuration
Remove ||= usage in ModuleBuilder
Document ModuleBuilder
Update Virtus documentation
ModuleBuilder specs

Test ModuleBuilder through Virtus.module
Add Configuration.build
Update spec style for Virtus.coerce and .coercer
Update configuration spec style