Convert to RubyGem format, change name to mesa_reader.

In preparation for minor name changes and moving to the RubyGems
platform, change the structure to be a functioning gem with
gemspec file. Also change main file name to 'mesa_reader' to
better conform to common conventions.
Conver mesa_reader to contain a module. Additional refactoring.

Now requiring mesa_reader merely loads the MesaReader module. Additionally,
the classes defined within MesaReader are MesaData, MesaProfileIndex, and
MesaLogDir (note that it's now always "Mesa" rather than "MESA").

Also refactored some methods in MesaLogDir to be cleaner and access instance
variables more indirectly. More to come.
Minor refactoring to MesaProfileIndex and MesaLogDir.
Refactor MesaData, mostly eliminating direct references to instance variables.
Update README's for recent changes.

Updated installation instructions and added information about the new module
structure as well as the new Class names/spellings. Also subtley moved away
from using any 'MESA_' spellings of things and moved towards 'Mesa'. For
example, MESA_Reader turns to MesaReader.
Fix gemspec file before pushing to rubygems.

Did not have tioga dependency properly stated.