holobase_document:find returns the pid of a document
Makefile: use eunit instead of common_tests
holobase_queue:tr_queue return a queue of transformation
holobase_queue:push add a new item to the queue

It also ensure that the queue have a fixed size
holobase_document: apply ops to the state

- Now a document hold a state.
- When an operation should be applied, we transform it if it conflicts with the
  ones already applied.
- To do so, the document hold a Queue of applied ops.
- The queue have a fixed size.
holobase_queue:tr_queue now excludes tagged ops

When pushing a new op, you tag it with the version of the document it is
applied to and a hash identifying the user who applied the op.
When creating a queue of transformation holobase_queue:tr_queue can now exclude
the ops from the queue that belongs to the same hash (i.e. user).
Document: filter the users's ops from the transformation queue

Now when creating a queue a transformation to update an op before applying it,
we filter the ops coming from the same user. This way, its modifications are
not doubly transformed.