Implement performance functions

Implement the performance functions, which is actually a list of values
to use together with the times values for the plot.
Create function to print to tikz

The times are scaled to be between 0 and 1, because tikz can't handle
very large values.
Merge matplotlib and tikz branches
Improves after merge

* The arguments for `__init__` of PerProfSetup is now just args
  (from argparse)
* Add option for logarithmic scale
* Add options for matplotlib and LaTeX/TikZ (they are exclusive)
* Create module for LaTeX/TikZ plot function
* Move examples from `tikzexample` to `test`
* Remove `tikzexample/profile.tex` since it was the output of this
  module
* Remove `tikzexample/examples.tex` since the user must know how
  to write it.

TODO:

* Add function to write the main document for the LaTeX/TikZ
* Implement the logarithmic scale option for matplotlib
Fix bug in README
Fix main.py and tikz.py

* In main.py the options `--mp` and `--tikz` not go into
  PerProfSetup
* tikz.py require math
* The plot method in tikz.py use generate_perf_functions
Implement log in matplotlib
Change --log to --semilog
Add flag to create header for tikz

Header consists of
- documentclass
- inputenc
- fontenc
- tikz
- pgfplots
- geometry