Add 'test' to setup.py + update README.md

I noticed the `bin/run_tests.sh` command line, but it is much easier for
me to just use the convention: python setup.py test

Add to setup() the test_suite=tests to register a test command
Add to README.md an "Hacking section" referencing the reference github
repository and explaining how to run the test suite.
Stop mentionning python2.6 support

reportlab requires 2.7+
discover does not seem to work for me with 2.6
Some of the code is using dict comprehension
Support for Travis CI

Since tests are using the cs_CZ.UTF-8 locale, we have to ask Travis to
generate it before running the tests. Related output is:

    $ sudo locale-gen 'cs_CZ.UTF-8'
    Generating locales...
      cs_CZ.UTF-8... done
    Generation complete.
    $
Make invoice.variable_symbol optional

According to https://en.wikipedia.org/wiki/Variable_symbol , the
Variable symbol is specific to the Czech republic.
Merge pull request #13 from hashar/optional-variable-symbol

Make invoice.variable_symbol optional
Merge pull request #12 from hashar/testsfix

Testsfix
Bump version