Don't call libexec commands directly

The rbenv command sets up paths in order to execute libexec commands itself.

Fixes first run issues as a result of https://github.com/sstephenson/rbenv/commit/cf2813600391341ea13d927667b1f08cb0769610.
Merge pull request #26 from mhoran/patch-1

Don't call libexec commands directly
Adding in jamie support.
Add integration tests for system install with a global Ruby version.
Rename system suite to system_ruby.
Update to README.
Update Gemfile.
Test system against 1.9.3-p362.
Test ubuntu-12.04 first.
Update CHANGELOG.
Reformatting CHANGELOG for 80-cols.
Release v0.7.2.

References #26
Up version for development.