Changed gains so they default to 1

Changed the default value of the gains so if no gain is set then the filters
will still try and behave as expected.
Added more common code to Filter

Added more common code to the abstract Filter class. Added a check for
non-zero positive numbers on setSampleRate().
Moved code around

Just moved code around so it comes off as easier to read.
Began work on implementing the filter

Began work on the update() function of this filter. Added some more class
member variables.
Updated README.md

Added information on the types of filters used. Expanded on some ideas in the
usage section.