support glob in bower main property

Support globing for files in main property of bower.json.
With that change we want eliminate dist directory, so no build is necessary and no dist is under version control. This avoids unnecessary conflicts and tags which contain the wrong version (if build forgotten).

See also bower/bower#935 and bower/bower.json-spec#30

Why did this line have to change? Previously: `return $.path.join(config.get('cwd'), file);`