Added filtering to galaxy.workflows.get_workflows().

I thought it would be nice to add filtering to galaxy.workflows.get_workflows(). Hope you find it useful too.

```
>>> gi.workflows.get_workflows()
[{u'name': u'bioblend-dev', u'tags': [], u'url': u'/api/workflows/4f2d50fb883583c7', u'published': False, u'model_class': u'StoredWorkflow', u'id': u'4f2d50fb883583c7'}, {u'name': u'Combine FASTA and QUAL', u'tags': [], u'url': u'/api/workflows/2491ee81eec9beee', u'published': False, u'model_class': u'StoredWorkflow', u'id': u'2491ee81eec9beee'}]

>>> gi.workflows.get_workflows(name='bioblend-dev')
[{u'name': u'bioblend-dev', u'tags': [], u'url': u'/api/workflows/4f2d50fb883583c7', u'published': False, u'model_class': u'StoredWorkflow', u'id': u'4f2d50fb883583c7'}]

>>> gi.workflows.get_workflows(workflow_id='2491ee81eec9beee')
[{u'name': u'Combine FASTA and QUAL', u'tags': [], u'url': u'/api/workflows/2491ee81eec9beee', u'published': False, u'model_class': u'StoredWorkflow', u'id': u'2491ee81eec9beee'}]
```