Added API documentation

- Applied the MediaGoblin docs theme to the API docs
- Added api-docs/build to .gitignore
- Configured api-docs/source/conf.py
- Added /api-docs/source/mediagoblin* to .gitignore
- Added apidoc directive to api docs Makefile, will build the API docs
  based on the source

.gitignore

@@ -13,6 +13,8 @@
 /mediagoblin.egg-info
 /docs/_build/
 /docs/build
+/api-docs/build
+/api-docs/source/mediagoblin*
 /user_dev/
 /paste_local.ini
 /mediagoblin_local.ini
@@ -31,4 +33,4 @@
 *.swp
 
 # The legacy of buildout
-.installed.cfg
+.installed.cfg