Fix default markers

[malmans2]

Close #37

Here is a PR with a solution for #37.
Feel free to close this if it's not the right solution!

[malmans2]

Part of the issue was that color "gray" can not be used.
Looks like matplotlib string color+marker only accepts single letters (i.e., matplotlib.colors.BASE_COLORS).

While I was looking at it I made a few changes:

1. colors and markers are now controlled by global variables, it should make it easier for users to change them
2. Combinations of marker-colors are repeated if needed (but a warning is raised when that happens)