Issue #9. Allow nested preconditions.

Due to my (probably bad) use of JAXB, simple precondition queries
syntax had to change in order to support the more complex case
of nested preconditions. They are now wrapped in:

```
<just><query>MY CYPHER QUERY</query></just>
```

AND/OR queries can now be defined and nested all together. The
following example is now fully supported e.g.:

```
<or>
	<and>
		<just>MY QUERY 1</just>
		<just>MY QUERY 2</just>
	</and>
	<just>MY QUERY 3</just>
</or>
```

Precondition executor is now more thoroughly tested, especially
with corner cases (invalid syntax, no column 'result').

Finally, a slight change on transaction management has been done:
there is no global transaction anymore, but one per changeset, as
it was initially intended.