Added libbe.version, wrapping the auto-generated libbe._version.

Added libbe.version, wrapping the auto-generated libbe._version.

Two major benefits:

1) Programatic access to the BE version.  Previously the
version-string logic was internal to the commandline interface ./be,
which left the other interfaces hanging.

2) Simple to override auto-generated revision id version with any
string you like (by setting libbe.version._VERSION, if that's the way
you want to run things or the way your packaging system requires.

Note that other modules should never read libbe.version._VERSION,
since it may not be defined.  Instead they should use the output of
libbe.version.version().