Twython unnecessarily disables compression.

While doing performance profiling as mentioned in lukasa/hyper#19, I stumbled upon the fact that Twython appears to send the header `Accept-Encoding: identity` (or leaves the header out altogether, it's not entirely clear, but they're semantically the same thing). This means that downloading the last 800 @-mentions to myself involves downloading 2.2MB of uncompressed JSON.

It seems to be trivial to turn this on by passing the appropriate header (`Accept-Encoding: gzip, deflate`) in `client_args`. More importantly though, this should be on by default when using requests. I've looked through the code and it doesn't seem to be deliberately turned off, so it looks like it's probably an accident.

From some code reading, the problem looks like it's around [here](https://github.com/ryanmcgrath/twython/blob/master/twython/api.py#L76-L83). In this block of code you end up ensuring that `Twython.client_args` always contains a `headers` key. Subsequently, in [this block](https://github.com/ryanmcgrath/twython/blob/master/twython/api.py#L110-L114), you ensure that `Session.headers` is _overwritten_ with your new `headers` dictionary. This overwriting gets rid of the default headers (set [here](https://github.com/kennethreitz/requests/blob/master/requests/sessions.py#L208)), which include the `Accept-Encoding` line.

I have two suggested fixes: 

1. Note that the headers will always be present and to treat them specially, by merging them instead of overwriting them.
2. Add `Accept-Encoding` to your default headers dict.

Sample code to reproduce the problem (this is set up for using `mitmproxy` to examine the HTTP headers: if you don't want to, remove the `client_args`):

```python
from twython import Twython

client_args = {
    'proxies': {
        'http': 'http://localhost:8080',
        'https': 'http://localhost:8080',
    },
    'verify': False,

# Uncomment this section to get the improved behaviour.
#    'headers': {
#        'Accept-Encoding': 'gzip, deflate',
    }
}

# Fill these in as needed.
API_KEY = ''
API_SECRET = ''
ACCESS_TOKEN = ''
ACCESS_SECRET = ''

t = Twython(
    API_KEY, 
    API_SECRET, 
    ACCESS_TOKEN, 
    ACCESS_SECRET, 
    client_args=client_args,
)

# Let's get all my mentions. First, get the initial set and save off the
# smallest ID found.
mentions = []
chunk = t.get_mentions_timeline(count=200, include_rts=1)
max_id = chunk[-1]['id']
mentions += chunk

# Now, keep looping until we reach the end or we've reached 800 tweets.
while len(chunk) > 180 and len(mentions) < 800:
    chunk = t.get_mentions_timeline(count=200, include_rts=1, max_id=max_id)
    max_id = chunk[-1]['id']
    mentions += chunk

# Print all the messages.
print("Messages: %d" % (len(mentions),))
print("Execution time: %f\n\n" % (end-start,))
```