README.md: improve formatting

* put Debian packages into copy-pasteable format
    * I think python-pycrypto means python-pycryptopp as I cannot find
    * python-pycrypto from [Debian
      repositories](https://packages.debian.org/search?keywords=python-pycrypto)
* quote flags and other commands
* replace backticks with `$()` in command nesting as is [generally recommended](http://mywiki.wooledge.org/BashFAQ/082)