Case importer cleanup

This PR is entirely code cleanup.

Largely addressing code style concerns by cleaning up extraneous spaces on lines, replacing tab characters with appropriate spacing and fixing indentation issues.

Also redid the column mapping table to move from using &nbsp; positioned table headers to using a properly formatted table (see changes in corehq/apps/importer/templates/importer/excel_fields.html).

Before:

![screen shot 2013-05-13 at 3 32 32 pm](https://f.cloud.github.com/assets/152134/497754/e05c51d2-bc03-11e2-91cc-a67ca1f3ee80.png)

After:

![screen shot 2013-05-13 at 3 28 33 pm](https://f.cloud.github.com/assets/152134/497751/d7e058be-bc03-11e2-8f5a-2019ac3cc2b6.png)