Make building from source easier

[b-]

Hi,

This PR changes the installer script's name to a more generic install.sh, changes the offline install process to allow installing straight from a git clone (i.e., we check for the presence of directory meta instead of directory offline, and then use $SCRIPTDIR as the repo directory), and includes a requirements.txt listing the libsass and watchdog packages the way pip install -r requirements.txt would require.

This way, an environment for hacking on this theme can be spun up and iterated upon as easily as doing the following:

1. clone the branch
2. install python3-pip
3. pip install -r requirements.txt
4. (le hac)
5. python3 sassy.py
6. sudo ./install.sh update
7. (moar hac)
8. python3 sassy.py
9. sudo ./install.sh update
10. [...]
11. git commit -a && git push