Adds a "rate" function to series, histogram, count and set types.
Introduces a breaking change to custom transforms by adding a second
parameter to define the amount of time over which the data was
collected.
Adds documentation on how to write transforms.
Fixed gregorian TTLs by normalizing to days
Merge pull request #48 from agoragames/rate

Adds a "rate" function to series, histogram, count and set types. 
Introduces a breaking change to custom transforms by adding a second parameter to define the amount of time over which the data was collected