Linux installation tutorial: make -j8

Added `-j8` flag to `make` in the [linux installation tutorial](http://docs.opencv.org/doc/tutorials/introduction/linux_install/linux_install.html).
I feel it's quite important for new users, who may otherwise spend a whole day just building the code!

Screenshot from built docs:
![doc_screenshot](https://f.cloud.github.com/assets/1893429/2157899/acb26e30-9488-11e3-98e2-502c51c941ac.png)