Changed to use a requests Session() object to enable pipelining

Reduced time for repeated quick operations by almost 40% on this trivial example:

```
for x in range(0,50):
    p.log(1.0, 7.2)
```

Before:
```
$ time python thisisatest.py
python thisisatest.py  0.21s user 0.06s system 2% cpu 10.562 total
```
after
```
$ time python thisisatest.py
python thisisatest.py  0.21s user 0.05s system 3% cpu 6.820 total
```