Script tag wins against global steal object

[pYr0x]

with stealjs/steal-tools#370 now css files should loaded correctly and baseUrl should be not overwritten
https://github.com/stealjs/steal/blob/master/src/config.js#L324

this fix some of the problems with
#578

[matthewp]

Hi, thanks for submitting. Given that #578 probably covers several different issues please describe specifically what this commit fixes. It looks like it is related to bundleSteal, what specifically does this fix?

[pYr0x]

talked to @matthewp
rewrite the PR. provide a test.
now we can write a global steal object like

<script>
    steal = {
        foo: 'bar'
    };
</script>

and steal merge that into the System-loader.

if a script tag attribute is provided, the property in System will be overwritten.
this is necessary because of this issue: stealjs/steal-tools#377
we now set

steal.baseURL = './';

on files where steal is bundled in. if the user would change the baseURL he can write into the script tag or config.