Added separate tests for structured, rectilinear, etc.
Fixed doctests.

Fixed
----
* Doctest for esmp grids needed to be fixed to reflect new ESMPy interface and
  and 'ij' indexing.
Updated package requirements.
Changed to define an empty VtkGridMixIn if no tvtk package.
Changed to read requirements from requirements.txt.
Changed to not install with sudo.
Changed to not run unit tests.
Removed import of unused warnings module.
Changed for PEP8 styling.
Changed to use new ESMF package and interface.
Removed blank last line.
Removed mappers subpackage imports.
Fixed imports for mappers subpackage.
Fixed syntax errors, PEP8.
Removed set_services function.

Removed
-------
* Removed the unused set_services function.

Added
-----
* New keyword, *if_exists*, for:
  - register_component_class
  - register_component_classes
  provides guidance on what to do if a class is already registered.
Changed to delete services before test setup.
Changed package setup for unit tests to use new methods.
Removed unused import statement.
Fixed unit tests that used xy-indexing.
Added nosetests command to Travis-CI.