Fixed Python 3 support

### Motivation
Running pingdom-python on Python 3 results in syntax errors at both install-time and run-time. After some deeper investigation, I found two problems.

##### 1. Invalid syntax
In Python 3, the following try/except block syntax is no longer valid.
```python
except PingdomError, e:
```
This must be changed to:
```python
except PingdomError as e:
```
This can be automatically done at install-time using the `use_2to3` switch in setuptools.

##### 2. Broken JSON parsing
The JSON parsing fails on Python 3.
```python
class PingdomResponse(object):
    def __init__(self, response):
        """Representation of a Pingdom API HTTP response."""

        self.headers = response.headers
        self.content = json.loads(response.content)
```
`response.content` returns a `bytes` object in Python 3. It returns a `str` object in Python 2. `json.loads` accepts a `str` object as an argument (as well as `unicode` on Python 2). I figured we could just use `response.json()` and let the `requests` package handle the JSON parsing (as well as encoding detection, since [UTF-8, UTF-16 and UTF-32 are all supported](http://tools.ietf.org/html/rfc4627#section-3)).

### Dropping Python 2.5
This brings up the question of Python 2.5 support. In **setup.py**, it is clear that pingdom-python depends on requests 0.10.8 or newer. As a side note, everything seems to be working okay with requests 2.1.0.
```python
      install_requires=[ 'requests>=0.10.8', 'simplejson'],
```
However, in requests 0.10.1, support for Python 2.5 [was dropped](http://requests.readthedocs.org/en/latest/community/updates/#id38). Unless Python 2.5 is a requirement, I would recommend dropping Python 2.5 support completely (as this pull request does). This means we can drop the simplejson dependency since Python 2.6 and newer have JSON support built-in.

----

:smile: Hi Mike.