Fixed unit tests for gradient sign convention.

Fixed
=====

* Fixes issue #1.
Merge pull request #2 from mcflugen/fix-gradient-name-change

Fixed unit tests for gradient sign convention.
Fix doctests that use old gradient function names.
Merge pull request #4 from mcflugen/fix-gradient-name-change

Fix doctests that use old gradient function names.
Fixed broken unit tests for structured_grid.
Merge pull request #6 from mcflugen/fix-utils-unit-tests

Fixed broken unit tests for structured_grid.
Merge pull request #7 from mcflugen/dev-docs

Updated the installation documentation
Added numpy-style docstrings.
Added numpy-style docstrings.
Moved module-level docs into class docs.
Added more detailed docs.

Added
=====

* More, numpy formatted, docs for landlab.field subpackage.
* New exception GroupError and FieldError.

Changes
=======

* Raise GroupError or FieldError instead of KeyError when trying to get
  a field that does not exist in the collection. For backward
  compatibility, both of these exceptions inherit from KeyError.
Fixed unit tests for new exceptions, name changes.
Added numpydoc extension for documentation.
Fixed numpydoc docstrings.
Added numpydoc module.
Moved numpydoc into extensions folder.
Removed extensions, add numpydoc to requirements.txt.
Fixed minor formatting errors.
Merge pull request #9 from mcflugen/fix-docs-for-field

Fix documentation for field subpackage.
Merge pull request #10 from mcflugen/dev-docs

Changed link to Developer's Guide