[FIX] fixed linux installer
[FEATURE] allow white spaces in molecular formulas
[FEATURE] isotope distributions with exact formulas if possible

    for R=None the isostope generator creates exact molecular formulas now
[FIX] fixed and improved linux installer

    virtualenv-2.7 was not found on some systems
    replaced this with call of virtualenf --python=python2.7
    added more checks and better error message
[INSTALLER] fixed requirements + docs