Allow for multiple alert handlers of the same type

[nathanielc]

Fixes #118

With this change each alert handler is appropriately scoped so you do not have to worry about name conflicts. You can now add multiple handlers of the same type.

For example:

stream
    .from().measurement('cpu')
    .where(lambda: "host" == 'serverA')
    .groupBy('host')
    .window()
        .period(10s)
        .every(10s)
    .mapReduce(influxql.count('idle'))
    .alert()
        .id('kapacitor/{{ .Name }}/{{ index .Tags "host" }}')
        .info(lambda: "count" > 6.0)
        .warn(lambda: "count" > 7.0)
        .crit(lambda: "count" > 8.0)
        .slack()
            .channel('#alerts')
        .slack()
            .channel('@jim')

[nathanielc]

Wait to merge this until after #113 so that we can rebase and integrate OpsGenie in the same way.