use exceptions instead of prints
This has the big benefit that we can use those methods even when we do not want to print something to stdout or stderr like in web interface.
fix config parser
add command to get and set a config value

Examples:

$ git tessera config user.email # get value of setting user.email
$ git tessera config user.email "tuxtimo@gmail.com" # set value of setting user.email
Merge branch 'master' of https://github.com/neolynx/git-tessera
Refactoring: create command for gittessera.
No title required for creating a Tessera.

When invoking
    git tessera create
no title is required, a tessea is created with the default
title.
correct path joining.
Refactoring: exported git interface abstraction.
Merge branch 'master' of github.com:neolynx/git-tessera

Conflicts:
	tessera/gitcommands.py
	tessera/gittessera.py
Make tests
Testing becomes better
First test runs.
tessera created: how do i override config editor setting for tests?