Added build-essential to the Linux requirements

A friend of mine was trying to install go1.2 on his Ubuntu machine, and the compiler complained about a `unistd.h` file not being found. The problem was solved by installing `build-essential`.

Hence, why I propose that we add `build-essential` as a Linux requirement. I'm certain a lot of people will be spared the trouble of trying to figure out where the error is coming from.