Periodic add

I implemented the clj-time.periodi/periodic-seq with an upper bound.

The implementation is very simple, I just add a `take-while` in front of the sequence that produces the values.

I have modify the docstring, to explain the changes, probably someone with a better English da mine should proof read it...

Also I have removed an useless dependecies in the namespace declaration.

Finally I have removed a type-hint, this last type-hint don't only looked useless (no warn-on-reflection triggered) but it seems to cause some problem in my environment.

I am not too sure about the type-hint thing, also I am not really sure to have understand why it raise an Exception, however I found this answer: 
http://stackoverflow.com/questions/15230061/cant-type-hint-a-local-with-a-primitive-initializer

The excpetion I get from the type hint is:
`clojure.lang.Compiler$CompilerException: java.lang.UnsupportedOperationException: Can't type hint a primitive local, compiling:(/home/simo/clj-time/src/clj_time/periodic.clj:6:1)`