Use net/http Request.Form for params instead of Params

The motivation behind this PR is to attempt to use the existing `Form` map in the http.Request struct instead of adding an additional parameter, `Params`, to the Handler function. This has the advantage of being compatible with handlers following the syntax of the `net/http` package while maintaining the ability to use URL parameters.

Using GitHub as the example from `go-http-routing-benchmark`; JHttpRouter can be seen to be slower and using a little more memory, there are fewer memory allocations. I feel the downsides may be an acceptable trade off for compatibility.

    BenchmarkJHttpRouter_GithubStatic   20000000    86.6 ns/op     0 B/op        0 allocs/op
    BenchmarkHttpRouter_GithubStatic    20000000    77.4 ns/op     0 B/op        0 allocs/op
    BenchmarkJHttpRouter_GithubParam    3000000     472 ns/op      180 B/op      0 allocs/op
    BenchmarkHttpRouter_GithubParam     5000000     340 ns/op	   96 B/op       1 allocs/op
    BenchmarkJHttpRouter_GithubAll      20000       82118 ns/op    19235 B/op    0 allocs/op
    BenchmarkHttpRouter_GithubAll       30000       60360 ns/op    13792 B/op    167 allocs/op

While I believe this may be at odds with @julienschmidt 's ideology for httprouter, I am still interested in their feedback and thoughts :)