Add generic MIN()/MAX() functions.
Implement support for sys.path when loading modules.

sys.path is not initialized by rt_init(), that's left for platform-specific
startup code. (For example, bare metal port may have some hardcoded defaults,
and let user change sys.path directly; while port for OS with environment
feature can take path from environment). If it's not explicitly initialized,
modules will be imported only from a current directory.
unix: Initialize sys.path from MICROPYPATH environment variable.

If it's not available, "~/.micropython/lib:/usr/lib/micropython" is used
as a fallback.
unix: Be sure to add current/base dir of a script to sys.path.

This mirrors CPython behavior and makes possible to run scripts which
import other modules not from script's directory.
Remove older import helpers, no longer used.
Merge pull request #262 from pfalcon/sys-path

Implement sys.path support