Merge pull request #1 from seajs/master

sync with seajs master
Merge pull request #2 from seajs/master

update
Fix the bug when src key is undefined,the shim plugin do not work.

When an alias item, the 'src' key is not defined, the code do not work
well with the exports value.
Merge pull request #653 from Rayi/master

修复Shim插件在未指定src时不工作的问题