Add Gemfile and Gemfile.lock
Use XDG spec for specifying default heliotrope dir
Add the lib directory to the LOAD_PATH automatically
Modify README to include usability improvements

* mailstore dir is now $XDG_DATA_HOME/heliotrope by default
* no need to -Ilib anymore
* use bundle instructions to install gems
Replace wmorgan with sup-heliotrope
Verify presence of lib dir before including it

When packaged, the lib dir might live in a completely different place,
yet be included in the $LIBPATH. This trick is only needed when
running out of any paths.
Check for heliotrope.rb existence

If the executable is in /usr/bin, checking for /usr/lib will return true
while it shouldn't.