Importing shapefiles into PostGIS (pure python)

### Import ESRI shapefiles to PostGIS database
#### Features
* Pure Python (no shp2pgsql)
* Shapefiles (and related files) are expected to be zipped archives, and not nested in sub-directories therein
* Coordinates are converted to WGS84 (ESPG:4326) so every table in the DB is consistent
* No integration with master table yet (and no centroid computation, but that's easy to do)

#### Location
* Added two functions in `wopr/models.py` that create a table schema from a shapefile
* Added two functions in `wopr/helpers.py` (didn't want to create another file) that carry out import and conversion

#### New dependencies
* shapely (geometries)
* fiona (shp I/O)
    - **note**: fiona in turns requires the GDAL/OGR python module, that in turns requires libgdal (and libgdal-dev)
* pyproj (crs conversion)
* json (useful as intermediate representation)
* zipfile

I've used pyshp before, but for some reason decided to go with fiona. The advantage of using pyshp would be to not have to carry out the hideous replacements at line 367, if I understand correctly.
The field type mapping in models.py assumes to receive a fiona collection, but I could adapt it if the benefits of using pyshp are worth it.
Thanks for the tip, I'll investigate this!