Main changes:
- switch from pyoro to pykb to be more conformant
- as a consequence, fix sloppy code + make dialogs more robust to KB weaknesses
- a couple of fixes in unit-tests

For information, results of 'dialogs_test all' with MinimalKB:

With minimalKB 0.4 (commit b86063e390a) + pykb (commit cea30f6f7ae) + dialogs(commit 20bbc2ec8467)
======================================================================
| suite            | nb tests | tests run | failures | errors ||  OK |
|--------------------------------------------------------------------|
| statements       |    63    |     63    |    26    |   11   ||  26 |
|--------------------------------------------------------------------|
| sentence         |    33    |     33    |     0    |    0   ||  33 |
|--------------------------------------------------------------------|
| discrimination   |    13    |     13    |     6    |    5   ||   2 |
|--------------------------------------------------------------------|
| parser           |    72    |     72    |     7    |    0   ||  65 |
|--------------------------------------------------------------------|
| verbalization    |    76    |     76    |     4    |    0   ||  72 |
|--------------------------------------------------------------------|
| questions        |    42    |     42    |    13    |    4   ||  25 |
|--------------------------------------------------------------------|
| TOTAL            |   299    |    299    |    56    |   20   ||  223 |
======================================================================
Total time: 54.577629sec