chore(readme): Example contains wrong name for time-frames and date-frames properties
bug(timeFrames): Rebuild columns after timeFrames or dateFrames property has changed
bug(headers): Add watcher for headers and headersFormats

If the headers or headersFormat settings change then this shall be reflected in the UI immediately
perf(header): Tiny performance improvement by caching the date value when generating headers

Instead of accessing moment for the current and previous column date value the current value can be cached and used as previous in the next iteration
bug(header): Use own watchCollection group for headers formats

Combining watch expressions with + will force angular to make a string out of the value. This makes it useless to watch objects as their string representation is simply [Object]