Filter1

Hi David,

Here is fourierShift, iFourierShift, and fourierFreq.
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can you comment on the signature for fourierFreq....
it's currently programmed so that you provide either an 
fs (sampling frequency) or a dt (time step). Both are -1 by default, 
and whichever you specify, the other is implied by 
     (sampling frequency) === 1/(time step).

Scipy takes only the dt argument, but fs would be more convenient, 
and if I were choosing just one, I would choose fs. 

The default values of -1 are kind of clunky, what do you think?
````
This closes issue [#127](https://github.com/scalanlp/breeze/issues/127)

Some basic tests for fIltering signals were added, as well as lowpass/highpass (filterLP, filterHP)

===========================

There seems to be **a big problem with my previous Pull Request** though... Slicing DenseVectors with Int seems to throw a compile error sporadically. The error goes away always if you comment out test/breeze/linalg/SliceVectorTest.scala lines 35 and 36.

The exact same code was passing fine, even Travis was OK with it, as you recall. I'm very worried
that I've broken something important. I'm not quite sure what's wrong, do you have an idea?
````
scalac: 
     while compiling: V:\docs\gh\breeze\src\test\scala\breeze\linalg\SliceVectorTest.scala
        during phase: specialize
     library version: version 2.10.3
    compiler version: version 2.10.3
  reconstructed args: -Xplugin:C:/Users/Kenta/.ivy2/cache/org.scala-lang.plugins/macro-paradise_2.10.3/jars/macro-paradise_2.10.3-2.0.0-SNAPSHOT.jar -language:_ -deprecation -classpath 
````