Add walkthroughs/tutorials for basic use-cases.

[jonathancrabtree]

Add walkthroughs/tutorials for basic use-cases to the documentation. The resulting images can also be added to the gallery.

[jonathancrabtree]

From the manuscript:

1. visualizing the gene annotation and DNA sequence data from a GenBank flat file e.g., for a finished or draft bacterial genome
2. investigating patterns of gene conservation in two or more closely-related strains or serovars
3. showing SNPS and indels in several microbial strains relative to a reference genome and reference gene set
4. viewing coverage plots of RNA-Seq data from multiple transcriptomic samples aligned to one or more prokaryotic or eukaryotic genomes

[jonathancrabtree]

A comprehensive GenBank flat file visualization tutorial has been added:

http://jonathancrabtree.github.io/Circleator/tutorials/gb_annotation.html

[jonathancrabtree]

Four done, only one (graphs and heat maps) left.