Express dependency between core and bc targets

```
$ git grep '\.\./' | grep '^[A-z]\+\/[A-z]\+:'
...
core/Makefile:117:include/bif.h: ../bc/scripts/bif.tab
core/Makefile:129:gentab/exp_tab.erl: $(PREMODS) ../bc/scripts/bif.tab
core/Makefile:130:      scripts/exptab_gen preload ../bc/scripts/bif.tab $@
...
```
Add build icon
Merge pull request #13 from lucafavatella/core-depends-on-bc

Express dependency between core and bc targets
Fix icon link
Use local nettle dep thus do not install package on TravisCI