[pl] translated first part of chapter 8
[pl] translation of section 8.1
[pl] translation of section about importing from Subversion repository
[pl] translation of section about import from Perforce
[pl] translation of sections about importing from other SCMs and summary
[pl] translation improvements in chapter 8
[pl] spelling fix in chapter 8
Merge pull request #458 from gibas/08-git-and-other-scms

[pl] translation of chapter 8