Add a Makefile with convenience commands

Some users can't install `httpie` into the global namespace, because of conflicts with dependencies like requests in other projects. This means those users have to do something like this:

```bash
mkvirtualenv httpie
pip install httpie
```

and then use a wrapper alias like this:

```bash
alias http=`~/.envs/httpie/bin/http`
```

Instead this adds a Makefile so you can clone the library and run `make install` to install it.

There are also helper commands for the tests:

```
make test-install
make test
```

I couldn't get httpbin to install by running "pip install -r requirements-dev.txt", I ran into the following error:

```bash
Scanning installed packages

Setuptools installation detected at /Users/kevin/code/httpie/venv/lib/python2.7/site-packages

Non-egg installation

Removing elements out of the way...

Renaming /Users/kevin/code/httpie/venv/lib/python2.7/site-packages/setuptools-0.9.7-py2.7.egg-info into /Users/kevin/code/httpie/venv/lib/python2.7/site-packages/setuptools-0.9.7-py2.7.egg-info.OLD.1379182538.52

Renaming /Users/kevin/code/httpie/venv/lib/python2.7/site-packages/setuptools into /Users/kevin/code/httpie/venv/lib/python2.7/site-packages/setuptools.OLD.1379182538.52

Renaming /Users/kevin/code/httpie/venv/lib/python2.7/site-packages/pkg_resources.py into /Users/kevin/code/httpie/venv/lib/python2.7/site-packages/pkg_resources.py.OLD.1379182538.52

Could not find the /Users/kevin/code/httpie/venv/lib/python2.7/site-packages/site.py element of the Setuptools distribution

Patched done.

Relaunching...

Traceback (most recent call last):

  File "<string>", line 1, in <module>

NameError: name 'install' is not defined
```

There's an issue with httpbin here that seems to get the same error: https://github.com/kennethreitz/httpbin/issues/106, but the corresponding PR was just closed with no comment so I'm not sure what the plan is. Anyway I got around it by cloning the library locally and running `pip install -r requirements.txt`.

I also had problems running the test suite locally. It seems that the test runner is looking for `HTTP/1.1 200` where httpbin responds with `HTTP/1.0 200`.