Document ExecRes and add methods for getting the trimmed output (that use a lazily initialized field). Use the method for getting the trimmed output everywhere it seems necessary. Only initialize author / committer info in the getCommitInfo() method if they are null.

doc of Repository methods and small refactorings to blameMergeConflicts (including renaming if to blameUnmergedFile)

# Conflicts:
#	src/de/uni_passau/fim/seibt/gitwrapper/repo/GitWrapper.java

# Conflicts:
#	src/de/uni_passau/fim/seibt/gitwrapper/repo/Repository.java

add the possibility of setting an environment for the executed process, attempt to force git to use english

Replace the conditional in the Commit constructor with a specialized DummyCommit class. This prevents accidentally setting any of the properties of the Commit.

Line based implementation of the 'git blame --porcelain' parser. Includes extraction of committer information, OffsetDateTime for both author and committer dates, storing the File associated with a line, and a Map holding all unhandled header fields.

closes #1

Correctly add new Commit objects to the commit map. put(...) returns the previously associated value leading to commits being overwritten.
Using computeIfAbsent(...) does not add Commits multiple times and prevents empty Optionals in the first invocation.