Add make targets for fast linting and testing
    
    Fast linting only needs modified files with pylint and diff with
    pycodestyle. It's good enough to detect most code errors very fast. It
    typically takes less than 10 seconds. A complete full pylint run uses
    all CPU cores for several minutes. PEP 8 violations are typically
    reported after 30 minutes to several hours on Travis CI.
    
    Fast lintings uses git diff and git merge-base to find all modified
    files in a branch or working tree. There is no easy way to find the
    branch source. On Travis the information is provided by Travis. For
    local development it's a new variable IPA_GIT_BRANCH in VERSION.m4.
    
    Fast testing execute all unit tests that do not depend on ipalib.api.
    
    In total it takes about 30-40 seconds (!) to execute linting, PEP 8 checks
    and unittests for both Python 2 and 3.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>