TEST Acceptance test to verify we preserve cover info

We should have the same report before loading a moka and after stopping it,
regardless of the calls done while in moked mode.

Note we are ignoring all calls done in moked mode. This is not the ideal
behaviour but is better than losing all the history so we will implement this
first and maybe look into counting calls in moked mode later
TEST unit tests for moka_mod_utils:get_cover_compiled_code
FUNCT basic support for cover compiled code in moka_mod_units

  * is_cover_compiled
  * get_cover_compiled_code (here be dragons)
DOC clarify a comment
FUNCT run the tests with and without cover

The code paths vary a bit in both cases
FUNCT restore the code for covered modules

Use the cover compiled code stored in the cover data base instead of reloading
the code from the beam file
TEST raise the timeout for the fsm test

It seems now that cover is preserved it is more likely to take more than 10
seconds in an i5.

I need to investigate more but it can be due to excessive bumping of the cover
db or because is harder to reload cover instrumented code ...
FUNCT update samerlib

Not needed but we want to keep dependencies up to date
DOC update a comment
FUNCT compile the tests to ebin

Most of the tools that are not rebar don't expect stuff to be compiled in
.eunit... plus we need to run the tests to get the files compiled if we don't
configure erlang like this

It sucks anyway
DOC Add documentation about cover in the README