Better error reporting

[X-Ryl669]

The idea here is to concatenate all warning found while using the code and spit out the warning at the end of the process once instead of for each warning found.

This also answers the question about "where" the warning happened (source file that triggered the warning), in addition to the current "what".
I'm capturing the source location in the front end and transmit it down to the warnings array.

This requires some patch in rename-css-selectors I'll commit as soon as the version is updated in the other PR.

This gives such report now:

WARNING: The following selectors were not found in the rename table, but appears in the compressed map. In order to avoid that some other selectors are used instead, they were appended with '_conflict'. You need to fix this either by:

- Creating a CSS rule with the selector name and re-run the process, or
- Excluding the selectors so it's not renamed, or
- Adding the value to the reserved selectors table so it's not used as a possible short name


The failing selector are:
 - 'v' found in:
    js/ex.js(98):     $('#nvsd ul.edit li input[name=v]').value(val);
 - 'l' found in:
    js/sensor.min.js(23): $('#lc').on('click', function(e){cancel(e);saveVal('l')...

WARNING: You shouldn't run this software on minified files as it'll be hard to debug errors whenever they happens.

[coveralls]

Coverage remained the same at 100.0% when pulling 5821dd7 on X-Ryl669:BetterErrorReporting into 2815d48 on JPeer264:master.

[X-Ryl669]

Ok, that's step 2 here. This use the PR from node-rename-css-selectors#44 to get source file name. Please merge this one before I can work on rebasing #95.