Use GEOS version instead of CAPI version (#56)

Parts of object names in geos.py and impl.py have been renamed:
 - `14` -> `300` for `3.0.0-CAPI-1.4.1`
 - `15` -> `310` for `3.1.0-CAPI-1.5.0`
 - `16` -> `311` for `3.1.1-CAPI-1.6.0`
 - `16LR` -> `320` for `3.2.0-CAPI-1.6.0`
 - `17` -> `330` for `3.3.0-CAPI-1.7.0`

`shapely.geos` has three version-related members:
 - `geos_version_string`, restored to prior versions of Shapely (still
   present in manual); this version string found in in geos_c.h and
   in other programs like PostGIS
 - `geos_version`, (major, minor, release) version tuple for GEOS library
 - `geos_capi_version`, former version tuple to determine GEOS library
   capabilities; example use of this member was removed from manual, as
   its use should be discouraged.

`geos_c_version` was an alias for `geos_capi_version`, but was removed.