Add support for local settings

By creating a local.mk, a developer can keep settings in a file instead
of typing them every build (such as ENABLE_WEBKIT2=no or PREFIX=...).
check if uzbl-event-manager is running before starting new instance

Essentially, uncomment the code that's already there.

Bug report:
http://www.uzbl.org/bugs/index.php?do=details&task_id=310&project=1
Merge pull request #103 from mathstuf/dev/allow-local-settings

Add support for local settings
Merge pull request #109 from veox/check-if-event-manager-running

check if uzbl-event-manager is running before starting new instance