Issue 100

Fix #100 
If ```ebi_study_accession``` and/or ```ebi_submission_accession``` are left as ```None```, then it will be inserted into the database as the string ```"None"```, whereas what we really want is ```NULL```, right?
```python
>>> test = '%s' % None
>>> print repr(test), type(test)
'None' <type 'str'>
```