Added liquid templating and migrated first agents
Use liquid in the matches section of EventFormattingAgent
LiquidMigrator is checking for complex JSONPaths

If a complex JSONPath like '$.text[*]' is found the migration will
fail with an exception and keep the database intact
Migrated the PushbulletAgent to use LiquidInterpolatable
Migrated JabberAgent to use liquid, added link to documentation
Migrated DataOutputAgent to use liquid
Migrated TranslationAgent to liquid
Migrated TwitterPublishAgent to use liquid
Made migration irreversable, support '$variable' syntax
Migrated HumanTask, Trigger and PeakDetectorAgent to Liquid

Made LiquidInterpolatable#interpolate_options recursive
Really interpolate the `message` options attribute of the PeakDetectorAgent
Merged all previous liquid migrations into one new migration
Added support for the `Agent#make_message` format to the liquid migrator
Merge remote-tracking branch 'huginn/master' into liquid-templating
Removed unused code
Merge pull request #285 from dsander/liquid-templating

Added liquid templating and migrated first agents