Output maf version number with "--mafversion"

Incomplete implementation for #89.

Currently maf.py obtain the revision number of git, but now maflib cannot access to it.
We have to implement core.ExpOptionsContext.__init__. The version is now a constant of in maf.py, but we cannot access this number from maflib. Maybe we have to make an auto generated file in .waf-*/maflib directory to save some constants.