color output and formatted xml

Adds color and formatted xml support through httpie.

This also includes the following non-cosmetic change. Interested in your feedback.

Form parameters must now go at the end of the command. This allows us to do this:
```bash
$ python jsonapi.py /Messages -X POST -d To=9197404420 From=9192386150 Body=Hi
```
instead of this:
```bash
$ python jsonapi.py /Messages -X POST -d "To=9197404420&From=9192386150&Body=Hi"
```
This also means that if you wrap the value of a key-value pair in quotes, those quotes will not be stripped--they will be sent as part of the value.