Check if the commit signature extraction is always reliable

[le-git-imate]

Description of issue or feature request
To create a signed commit, the signature is computed over the commit fields. Then it is extracted from the commit by using a specific string pattern. Next the commit and its signature form a signed Git commit which will be used to create a signed Git commit object.
We need to make sure that the signature extraction always works.
This issue could be resolved in accordance with #3.

Current behavior

• le-git-imate uses a very basic approach to extract the commit signature which may fail in some corner cases (if there is any!).

Expected behavior

• Add unit tests to make sure commit signature extraction function works correctly.