Fix some errors on README and add Linux instructions

This can be split if you don't want the Linux patch. But it looks pretty
unintrusive to me.
Rebase: Week 1+2 graphical limits: Change stated epsilon from fraction to decimal

Since only decimal answers are accepted by the grader, using a fraction as
an example is confusing.

Update: This has been rebased and now fixes all instances of this confusion in
Weeks 1&2.
Merge pull request #67 from mateor/dev1

Fix errors in and update README; Adjust bad data type in example text