Allow callback to be optional with pluginify

This fixes the shim to make the callback argument to be optional. For
example, this is valid steal:

```js
steal('can', 'app');
```

Fixes #31
Merge pull request #31 from matthewp/no-callback

Pluginify shim should allow callback to be optional
Prevent swallowing of errors

Previously we were swallowing errors that occurred during the
multi-build process. This adds a test of what happens when we try to
load a module that doesn't exist. The result should be that the promise
is rejected, bubbling all the way up to the user. This fixes it. Fixes
Minify globals prior to transpiling

This fixes #65 by performing minification on globals prior to
transpiling.
Merge pull request #64 from matthewp/errors

Prevent swallowing of errors
Exclude plugins for global pluginified files

This adds an option to concatSource to allow the exclusion of plugins
from the bundle.source. This is necessary because o plugins that are not
meant to be included in the build, a `System.set` call is added which
will not work for global pluginified outputs. Fixes #42
Merge pull request #63 from matthewp/multi-build-errors

Fixes for multi build errors
Merge pull request #62 from matthewp/exclude-plugins

Exclude plugins for global pluginified files
Test dev code removal for System.defines

This tests System.defines that are minified to be sure that the
steal-remove-start/stop code is also removed.
Remove development code in the multi-build

This adds the ability to remove development code from the multi-build
using the same configuration option provided by pluginify. Fixes #67
Merge pull request #66 from matthewp/global-minify

Minify globals prior to transpiling
Merge pull request #68 from matthewp/multibuild-clean

Remove development code in the multi-build
Upgrade UglifyJS

We need to use the tilde ~ to select the version of UglifyJS to use.
Previously we were using the exact version number which caused problems
because that version was no longer in npm. Using ~ will keep us up to
date on all patch releases without upgrading to the next minor release.
Fixes error reporting test

Fixes a test that reports an error when attempting to build a module
that isn't in the filesystem. Test was working fine but because Steal
does a console.log on errors it was sending a File Not Found message
into the console that could be misinterpreted. Fix is to swallow
console.logs during the running of the test and set it back afterwards.
Merge pull request #72 from bitovi/error-swallow

Fixes error reporting test
Update to latest transpile