Merge pull request #2 from jschwab/master

Miscellaneous Code Cleanup
Restore the "include" statements to include the Dobjects module in each class so Dvectors can be easily instantiated (previously broken if the module had not already been loaded).
Re-remove include statments for Dobjects module but fix calls
to Dvector class methods to Dobjects::Dvector, helping preserve
the namespace.
Made MESADate#where and MESALogDir#select_models accept an arbitrary number of
keys (rather than just one). For every key you provide, your block must accept
an additional argument, and your test may now make use of more than one type of
data as a result. Also update the README to reflect these changes.
Add warnings to MESALogDir#select_models that trigger if the provided keys
don't work or if there is no block given.
Add magic methods to MESAData class.
Remove old implementation of MESALogDir::select_models
Update README with magic methods information.
Make magic methods access header data, too.
Refine magic methods part of README to reflect inclusion of header data.
Update "Last Edited" information.
Fix "filename" in MESAData section in README.

Was supposed to be "file_name", which is an actual method.