Parameterize Counter on the numeric type

Closes #5.

This is a breaking change which adds a second type parameter to
Counter, which is used to actually count the values. Some existing
code may fail due to the new required parameter. In particular,
type discovery appears not to be able to correctly determine that
N should be usize by default when creating a counter via init.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "counter"
-version = "0.3.3"
+version = "0.4.0"
 authors = ["Peter Goodspeed-Niklaus <peter.r.goodspeedniklaus@gmail.com>"]
 description = "Simple package to count generic iterables"
 repository = "https://github.com/coriolinus/counter-rs"
@@ -10,3 +10,4 @@ keywords = ["count"]
 license = "MIT"
 
 [dependencies]
+num-traits = "0.2"