Fix mibtool to allow for parallel builds

For some reason on linux, invoking -j5 on mibtool (not mibtool.py)
caused the program to hang and need to be -9 killed.  Traced the
problem to running mibtool.py via an import mibtool call.

Replaced it by importing the MIBTool class and running it directly.
Merge pull request #63 from timburke/pr-mibtool-fix

Fix mibtool to allow for parallel builds