Remove automatic adding of theme options to database

See #110
Use `thematic_default_opt()` in theme options upgrade check

See #110
Parse in default theme options in `thematic_get_theme_opt()`

Use wp_parse_args() to use default theme options even when no
options have been saved to the database yet.

See #110
Use thematic_get_wp_opt() in thematic_default_theme_layout()

Previous use of thematic_get_theme_opt() caused an infinite
loop since thematic_get_theme_opt() now fallbacks to
thematic_default_opt() which calls thematic_default_theme_layout().

Using thematic_get_wp_opt() ensures we actually check if database
option exists or not.

See #110
Merge pull request #112 from ThematicTheme/feature/default_options_not_in_database

Feature/default options not in database

Close #110