Touched up the README.md a little

Replaced all inline code blocks with proper GFM codeblocks
Removed `bash` from the beginning of commands (that's what the shebang in the files are for) and `>`'s
Added package manager notice to dependencies
Added version requirements of SFML and sparsehash to README.md
Merge pull request #79 from kyrias/master

Some README.md touch-ups