Expand the path when creating new projects

Fixes an issue where if you supplied a relative path when creating a
project, all of the relative references from that point on were
incorrect. By expanding the path on project creation, we ensure that
we're always setting the path the proper way.

Fixes #121
Add note about path expansion to documentation
Merge pull request #124 from CocoaPods/gf-relative-project

Expand the path when creating new projects