renamed output temporary test directory
set explicit use of unittest objects
moved get_dihedral() function to PBlib
renamed output temporary test directory (2)
created tests for functions
Ignore macos .DS_Store files

When the MacOS finder visits a directory it tends to create a file
called .DS_Store that describe how the directory should be displayed.
These files should not be viewed by git as they are just polluting the
repository.
Merge pull request #11 from jbarnoud/ds_store

Ignore MacOs .DS_Store files
create read_pb_definitions() in PBassign.py