Compute response of filters and test against matlab freqs and freqz

Function to compute the response of filter types both digital and analog.
I hope this fits in with the current project.
Any comments or changes would be appreciated.

The exported functions are 
```julia
function response(tff::TFFilter, is_digital::Bool, w::Number)
function response(tff::TFFilter, is_digital::Bool, w::Array)
function response(tff::TFFilter, is_digital::Bool, hz::Number, fs::Integer)
function response(tff::TFFilter, is_digital::Bool, hz::Array,  fs::Integer)

function response(filter_type::Filter, is_digital::Bool, hz::Array, fs::Integer)
function response(filter_type::Filter, is_digital::Bool, w::Range)
function response(filter_type::Filter, is_digital::Bool, hz::Range, fs::Integer)
```

A few issues that might be worth discussing:

1. I needed to export the Filter type from filter_design.jl. Was this not exported for a reason?
2. This meant I had to use rexport to pull in the filter using the following code, that throws the warning
````julia
include("filter_design.jl")
using Reexport
@reexport using .FilterDesign

>> Warning: using FilterResponse.FilterDesign in module DSP conflicts with an existing identifier.
```



Also I included tests against the matlab first examples for freqs and freqz. 
You can also recreate the figures from these examples as shown below, code is in the test file.

http://www.mathworks.com.au/help/signal/ref/freqs.html
![matlab-freqs-mag](https://cloud.githubusercontent.com/assets/748691/3146285/acfc26c6-ea44-11e3-8968-1489856bb111.png)
![matlab-freqs-phase](https://cloud.githubusercontent.com/assets/748691/3146287/ad205c6c-ea44-11e3-9b93-867dc70434c2.png)

http://www.mathworks.com.au/help/signal/ref/freqz.html
![matlab-freqz](https://cloud.githubusercontent.com/assets/748691/3146286/ad10149c-ea44-11e3-8794-b862fe52ccee.png)