Add free format

Add `--free-format` flag that only care about the problems that
converge be mark with `c`.
Fix some problems

Thanks @abelsiqueira for the review.
Remove dict.keys() when not need it

In a for loop over a dictionary we don't need to use the `keys`
methods.

Thanks very much to @gvwilson for raise the same problem in
another pull request.
Add option to limit the x-axis range

In most cases you won't the information for high performance
ratio because no one like to wait more than 100 times of the best
solver time.

* Add option ``--tau``
* Change TikZ backend for use the new option

NOTE: This is just a hack. There must be a better way of doing this.
Add labels to TikZ output

Add xlabel and ylabel for the TikZ output.

TODO:

* Add translation of the label
* Add options to change label
Update README
Change pdf to standalone latex header
Add PGFplots compatibility to 1.9
Merge pull request #44 from lrsantos11/pgfcompat

Add PGFplots compatibility to 1.9
Merge remote-tracking branch 'lrsantos/standalonepdf'

Conflicts:
	perprof/tikz.py
Add option to pgfplot compatibility

@abelsiqueira request low the value of pgfplot compatibility option.
Low the value is not the best solution due it is dependent of the
installation.

To solve the problem we add a new option in the command line. If the
user don't use it perprof-py will be compatible with the newer version.

A brief explanation about this solution can be found at
http://tex.stackexchange.com/a/81912/22451.
Merge remote-tracking branch 'raniere/free-format'
Merge remote-tracking branch 'raniere/keys'
Merge remote-tracking branch 'raniere/pgfcomp'
Merge pull request #41 from r-gaia-cs/rename

Update README
Merge raniere/tau and fix empty tau
Merge raniere/labels first commit
Fix bug when count the number of problem solving

Some solvers can save the time spending solve the problem as 0.0
and this will break the scale due a division by zero. We solve
this problem using a regex to check if the time is zero and if it
was adding a epsilon.
Raise error if time spending is zero
Hide pdflatex output but add flag to show it