Support line graphs

This fixes #87.
Use @ in haddock comments
Have some default graphDataColors

So that people actually see something when they use the
defaultGraphConfig. The colors can probably be improved.

The use of an infinite list here is justified: We are always passing it
to a `zip`-like function which will only use as many as there are data
values provided by the user.
Draw the graph frame pixel-crisp

With cairo, if you want to draw a one-pixel-wide line, you need to put
the coordinates in the middle of the pixels, hence the -0.5 and +1.
New Graph option: graphBorderWidth

The main purpose of this is to be set to 0 to disable the border. But
who knows, maybe someone likes thicks borders...
Also make the vertical bar widget border pixel-exact
Merge branch 'master' of git://github.com/nomeata/taffybar into nomeata-master
Remove commented-out code