EUnit plugin

Running `make tests` will now also run EUnit tests.

Users can set different `ERLC` and `EUNIT` options by providing the `EUNIT_ERLC_OPTS` and `EUNIT_OPTS` variables, respectively.
@efcasado thanks for your work on this we are super close now. If `EUNIT_DIR` is blank, FreeBSD & OSX complain:

```
GEN    clean
 GEN    clean-app
 GEN    clean-ct
 GEN    clean-eunit
 ERLC   testy.erl
src/testy.erl:0: Warning: function fu1_test/0 already exported
src/testy.erl:0: Warning: function fu2_test/0 already exported
 APP    testy.app.src
 GEN    build-ct-suites
 GEN    tests-ct
find: illegal option -- n
usage: find [-H | -L | -P] [-EXdsx] [-f path] path ... [expression]
       find [-H | -L | -P] [-EXdsx] -f path [path ...] [expression]
 GEN    eunit
======================== EUnit ========================
directory "ebin"
  module 'testy'
    testy: fu1_test...ok
    testy: fu2_test...*failed*
in function testy:'-fu2_test/0-fun-0-'/1 (src/testy.erl, line 13)
**error:{assertEqual_failed,[{module,testy},
                     {line,13},
                     {expression,"fu ( )"},
                     {expected,wu},
                     {value,shu}]}


    [done in 0.006 s]
  [done in 0.008 s]
=======================================================
  Failed: 1.  Skipped: 0.  Passed: 1.
erlang.mk:1020: recipe for target 'eunit' failed
gmake: *** [eunit] Error 1
```