Improve genome visualisation module

[js51]

This was thrown together quite quickly and could do with some improvements:

• More options for representing genomes (currently they need to be converted to a position paradigm (reg->pos) list
• Option to save image files with transparency
• Option to move arrows to the ends of each region
• Option to have a gap instead of a line separating regions
• TikZ output (probably a complete rewrite)
• Option to change output size
• Label sizing options
• Export images that inherit fonts from LaTeX document rather than using embedded ones #31

[js51]

I also hear that networkx has some interactive visualisations. Could be fun!