doctesting Hawk.hs

the normal doctest arguments are not enough, because Hawk.hs depends on
a normally-unavailable module Paths_haskell_awk generated by cabal.
A system to test the README's commands.

There used to be a bug somewhere which was causing doctest to fail with
a strange "hClose" error whenever the ByteString modules were indirectly
included by one of the test files. After clearing my sandbox, upgrading
cabal's package list, and reinstalling, the test now passes just fine.
refactor common arguments in doctest call
a better way to detect the sandbox used

The old method was not working with the new doctests.
test README's custom prelude example.
last README example, without an input file
test a few examples from the documentation
test all remaining examples from the documentation

Ideally, those tests should be extracted automatically from the README
and the documentation.

some of the tests are disabled, because they fail.
remove trailing whitespace
fix example from documentation

echo doesn't interpret escape sequences, printf does.
fix doc and tests: ByteString must be qualified.
remove unused imports
Merge branch 'issue-14' into develop