Revert "moved call to pluginsDir() out of header to fix compile #352"

This reverts commit 6c8f896f28ef50b124827b76eb83f7545599ce38.
Revert "added a plugin autoloader #352"

This reverts commit b740d650797003f0dc5652acf373b0948bb906e9.

Conflicts:
	src/mongo/plugins/autoload.cpp
removed superfluous admin db check #367
gave PluginHandle a destructor so it's exception-safe #367
restricted loadPlugin to take the name of a plugin only #376
restricted plugins to only be loaded from the plugins dir #376
added Loader::autoload to load a list of named plugins #376
added --plugins arg to mongos and mongod to autoload plugins #376