Setting for specifying a "github_base_url".

This is just the github version of stackConfig.git_url.
Merge pull request #49 from oremj/add_github_base_url_setting

Setting for specifying a "github_base_url".
Bail with usage message if config does not exist.

Catch and handle the exception which occurs if -c
does not exist.
Merge pull request #50 from oremj/handle_no_config

Bail with usage message if config does not exist.
Guard against 'null' actions when determining lastBuiltRevision.
Merge pull request #51 from racker/fix_jenkins_null_actions

Guard against 'null' actions when determining lastBuiltRevision.
Add support for toggling notifications for nagios servicegroups

Support toggling alerts by nagios servicegroups as well as hostgroups.
Also updated log messages to indicate whether alerts are being disabled
for a servicegroup or hostgroup.

Lastly updated comments to reflect that host notifications are NOT
disabled along with service notifications.
Added comments to nagios lib

Added comments to show where the "magic numbers" used in the nagios util
lib come from.
Rename missed variable

Rename variable hostgroup to servicegroup in the toggle notifications by
servicegroup function.
Add parallelLimit to knife queryAndRun

Add ability to limit the number of concurrent tasks run against the
servers knife returns.
Support functions as parallelLimit for knife.queryAndRun

Support functions as the value for parallelLimit so users can limit the
number of tasks run in parallel based on the number of servers found.
Implement suggestions from PR

- Rename parallelLimit to parallelLimitFunc and expect value to always
  be a function.
- Do not return whatever async.forEachLimit returns, we don't need it.
Abort queryAndRun on failure when using parallelLimitFunc

Need to be able to abort queryAndRun when a failure occurs while
remaining backwards compatible with anything currently using
queryAndRun. This commit enables this ability only when the
parallelLimitFunc option is used.

Because of the version of the async library being used, queryAndRun must
be, and can only be aborted when parallelLimitFunc is enabled.
implement fixes suggested in PR
Merge pull request #52 from racker/dominic/support_nagios_servicegroups

Add support for toggling notifications for nagios by servicegroups
Merge pull request #54 from racker/dominic/queryAndRun_parallelLimit

Add parallelLimit to knife.queryAndRun
Merged upstream updates