Improved instructions for debian-based installs

[danpoltawski]

I know that you are keen to get people to use the pip package, but this was roughly my experience doing it on debian:

$ pip install moodle-sdk
bash: pip: command not found
$ apt-cache search pip
<eventually find the right package>
$ apt-get install python-pip
$ pip install moodle-sdk
<mysql errors I didn't realise the cause of>
<googling>
$ apt-get install libmysqlclient-dev
$ pip install moodle-sdk
<postgres errors, I spot a pattern>
$ apt-get install libpq-dev
$ pip install moodle-sdk
<python errors>
$ apt-get install python-dev
$ pip install moodle-sdk
<hurray, it works!>

You could shortcut this by making the instructions for debian-based installs be:

$ sudo apt-get install python-pip libmysqlclient-dev libpq-dev python-dev
$ sudo pip install moodle-sdk