Allow running emsdk_env.sh from another directory.

Move into the emsdk directory before running ./emsdk. Make it possible to add `test -x $EMSCRIPTEN/emsdk_env.sh && source $EMSCRIPTEN/emsdk_env.sh` to my shell initialization script.
Merge pull request #15 from warrenseine/master

Allow running emsdk_env.sh from another directory.