Added code to reuse the Http connection when using httplib2.Http object

I was running into problems with an app that made thousands of Solr queries.  Many sockets were being openned, used once and then left open.  This ended up causing: "[Errno 99] Cannot assign requested address".

if I ran this code, I was able to confirm this behavior using netstat (netstat -ptn  | wc -l).

```python
import pysolr
conn = pysolr.Solr(url)
while True:
    res = conn.search(query)
```
After my changes, running the same code and netstat showed that only a fixed number of sockets were being created and the Exception mentioned above did not occur.