Add unit tests for mch_isdir.
Remove static declaration of executable_file.

Since static functions are only visible in the file in which they are
defined, this needs to be done in order to unit test the function.
Add unit tests for executable_file and mch_can_exe.

Caution: There is a pending test which should be implemented.
Port mch_isdir to libuv.
Implement pending test.
Remove USE_SYSTEM and code only run under USE_SYSTEM
Issue #66 - Add lint tool and uncrustify config. Update contributing documentation on coding style.
Fix style issues.

* Rename mch_full_name to mch_get_absolute_path.
* Rename mch_is_full_name to mch_is_absolute_path.
* Add a lot of missing parentheses.
* Remove yoda-conditions for consistency.
* Remove spaces in function declaration.