Enable empty prefixes for inlined embeddable

This fixes http://www.doctrine-project.org/jira/browse/DDC-2987

This makes it possible to map a field from an embeddable to a database field with the same name, without any prefix added.

Example:
- an embeddable object "Id" with a property "id"
- per default this would map inline to id_id
- supplying null or '' as columnPrefix does not work due to the ! empty() check
- with my little change, if columnPrefix : false is supplied in the mapping config this will now map to a db column "id"

To build Ids as ValueObjects is a very common approach in DDD, so ihmo this is a must have.
columnPrefix must support string and boolean, so changing to mixed
Test are for fix of ticket DDC-3028

Added 3 tests that check regression for:
- columnPrefix is empty
- columnPrefix is a string
- columnPrefix is false

Added some small entity / embeddable classes to support the test cases.
This fixes ticket DDC-3028

Enables columnPrefix to be "false" so no prefix is added.
Changed the structure a bit (to if/else) to be more readable with the additional condition.
Merge pull request #978 from AntonStoeckl/embedded-empty-prefix

[DDC-2987] Enable empty prefixes for inlined embeddable