Adding query filters

### Summary

I'm adding some basic string query filters to the api. This closes #88 
I'm also modifying the way /issues internally. I separated the labels into a separate `Label` model in order to easily query them later on.

#### app.py
- Add `Label` model
- Modify `labels` field of `Issue` model
- Modified `pages_dict` and `paged_results` to consider querystrings.
- Added `get_query_params` function to parse a querystring into a dict.
- Modified `get_organizations` and `get_projects` to filter Organizations and Projects using querystring paramenters (the actual filter is done using `like`)
- Modified `get_projects` to also search using querystrings like `organization_type`
- Added `/issues/labels/<labels>` to filter issues by a comma separated list of labels
- Added `/organizations/<name>/issues` to get all issues of projects belonging to a particular organization

#### run_update.py
- Modify `get_issues` to also return the issues labels as a list of dicts
- Modify `save_issue_info` to also receive a list of labels, convert them to Models and correctly associate the labels with it's corresponding issue.

#### tests.py
- Added `test_issue_with_labels` test for /issue/labels
- Added `test_organization_query_filter` tests
- Added `test_project_query_filter` tests
- Added `test_organizations_issues` tests