Implement support for standard repo layouts and other usability enhancements.  Fixes #14 from @jamieechlin.

``evnadmin create foo`` will now automatically create the standard repo layout
by default if the 'standard-layout' config variable is set and the config var
'no-svnmucc-after-evnadmin-create' is not set.

The default behavior should still result in acceptable behavior even when
enabling enversion against existing repositories with non-standard layouts
because we only intercept the following conditions:
    - svn mkdir ^/foo
    - svn rm ^/trunk (we let an rm of ^/foo go through)
    - direct replace of ^/trunk (hard to do this via command line)

Also, fix ``evnadmin enable`` such that it always runs the repo analysis
first; without this, you could wedge a commit if you disabled, committed, then
re-enabled without analyzing.