Rework README to include the list of online resources

Merges doc/SITES into the README file and moves the installation
instructions to their own file.
Fix submodule-related setup to properly check setenv return value

Commit 7fdd95a switched from using internal putenv-based setenv
work-around to setenv from libiberty. The internal work-around returned
non-zero on success whereas setenv returns zero on success, which means
that tig exits with the following error when started inside a submodule:

	tig: Failed to set GIT_WORK_TREE to ...

Fix calls to setenv to check for non-zero return value and correctly
catch errors.

Fixes #188