Updated INSTALL

- Added more detailed instructions for compiling on Linux.
- Removed markdown, and reverted to plain text
- Column width at 80 max (except for single command in Mac section, which existed prior to my editing)
- Removed references to build support for opus, webm, and vpx
- Noted FDK-AAC as optional to install
- Added instructions to install from PPA
- Added package list for RPM-based distros
- Moved source directory from /build to ~/source