Remove hard dependency on bootstrap

[ameyms]

Although lightning has a dependency on bootstrap, there should be no compile time dependency. For example, right now, we depend on button-type mixin and so bootstrap needs to be a part of the build. Further, since bootstrap is imported, compiled css also contains all of bootstrap code.

Should we only import mixins?
Is is ok to compile the whole thing?
Should we stub/'polyfill' the required mixins?

[rahejar]

It doesn't seem right to compile the whole thing as it just repeats the bootstrap code. Importing only mixins looks fine but can't a mixin depend on some other bootstrap code and therein creating a dependency chain?