Prevent tsplot to require a X Server

According to [this answer on StackOverflow](http://stackoverflow.com/questions/4931376/generating-matplotlib-graphs-without-a-running-x-server) doing this `import` and `use` statement will prevent pylab from requiring a running X Server. 

I tested it briefly on a ubuntu virtualbox and it seems to work.
Merge pull request #103 from mweibel/patch-2

Prevent tsplot to require a X Server