Added ability to load libraries from an external source

Added loadnExternalLibrary method to the import process which
will check an optional Sk.externalLibraries config for info on
where to find the library
Fixes default value cases
Fixes firefox issue regarding precision error
Refactors ff precision fix
Merge remote-tracking branch 'upstream/master'
Adds missing test case for closure defaults #236
Revert "Refactors ff precision fix"

This reverts commit 46a5bdb9fe95f83d1e45e90470dd8e3c51900733.
Revert "Revert "Refactors ff precision fix""

This reverts commit 6fe8c37009fff6d0c2288cbeb956af8e316e7e6f.
Strange binary diff error
Merge pull request #240 from rixner/master

Preserve cell variables across calls to yield.
Merge pull request #239 from waywaaard/precision_format

Fixes firefox specific bug for precision parising in str.js
Merge pull request #237 from trinketapp/master

Added ability to load libraries from an external source dynamically at compile time
pre- rename conflicting test files
Merge branch 'pr' of github.com:waywaaard/skulpt into waywaaard-pr
Merge branch 'master' of github.com:skulpt/skulpt