Feature/transformer guessers

Bug fix: [no]
Feature addition: [maybe]
Backwards compatibility break: [yes]
Unit test passes: [yes]
Behat scenarios passes: [yes]
Checkstyle issues: [no]*
ChangeLog updated: [no]
Fixes the following JIRA: maybe...

This PR removes all need of configuration for import Transformers by adding Guessers which automatically choose the right PropertyValueTransformers according to the column's label and the doctrine metadata for the imported class.

The current set of guessers classes should be sufficient for treating most cases. The most interesting guessers are :
  - the regex guesser which guesses by applying regexes to the label of the column
  - the type guesser which guesses according to the type of the doctrine column related to the label
  - the atttribute guesser which guesses according to the related attribute backend (only for products)

This PR also features :
  - an OrmTransformer which can be used with any entity 
  - a specialized ProductOrmTransformer
  -  a TransformerProcessor class wich can be used for any entity. 

The association processor has already been removed in favor of a TransformerProcessor. Other processors will be removed in the future.