Fully integrate GenomeInfoDb

* Fixes #20 
* Removes gc() calls as recommended by http://adv-r.had.co.nz/memory.html
* Removes some rm() calls near the end of functions.
* Adds an option to regionMatrix() to normalize coverage prior to filtering.
* Implements start up tips a la ggplot2 style.
* Minor changes to some functions to make them marginally more efficient.