Now the NxsGetter object has responsibility for all I/O, rather than
pycURL. It will choose whether the data pycURL retrieves is sent to an
in-memory buffer, or if it will be written to disk, as appropriate.

This change is needed so that we can minimize the number of curl
instances. Keeping only one handle means that curl can better handle
keepalives and other gritty networking details. The docs for libcurl encourage
this approach.
Messed up curl's cookiejar and cookiefile options last commit. Fixing.