[core] Refactor engine

This is going to be a bit more comprehensive than anticipated. In order
to cleanly refactor the core and the engine, basically start from
scratch with the implementation.

Goals:

* Test coverage
* Maintain backwards compatibility with module interface as much as
  possible (but still make modules easier to code)
* Simplicity

see #23

bumblebee/util.py

@@ -1,33 +0,0 @@
-import shlex
-import subprocess
-try:
-    from exceptions import RuntimeError
-except ImportError:
-    # Python3 doesn't require this anymore
-    pass
-
-def bytefmt(num):
-    for unit in [ "", "Ki", "Mi", "Gi" ]:
-        if num < 1024.0:
-            return "{:.2f}{}B".format(num, unit)
-        num /= 1024.0
-    return "{:05.2f%}{}GiB".format(num)
-
-def durationfmt(duration):
-    minutes, seconds = divmod(duration, 60)
-    hours, minutes = divmod(minutes, 60)
-    res = "{:02d}:{:02d}".format(minutes, seconds)
-    if hours > 0: res = "{:02d}:{}".format(hours, res)
-
-    return res
-
-def execute(cmd, wait=True):
-    args = shlex.split(cmd)
-    p = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
-
-    if wait:
-        out, err = p.communicate()
-        if p.returncode != 0:
-            raise RuntimeError("{} exited with {}".format(cmd, p.returncode))
-        return out.decode("utf-8")
-    return None