Add a couple new source map features

This adds 2 sourcemaps features that will allow Sentry to work with the source maps generated by browserify:

1. Loading data URI source maps.  The allows you to include your source map as a base64 data uri inside you generated source to avoid having to have a publicly available source map in a seperate file: mdn.io/data-URIs

2. Parsing inlined source content.  The v3 spec for source maps adds the `sourcesContent` field, which can contain an array of strings which contain the contents of the original source files in the sources field. see line 6 of https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit?pli=1#heading=h.qz3o9nc69um5

I have not been able to run the test on my machine, so this PR is still a work in progress, and I am submitting it more to start the feedback process than as a final product.  I haven't really written any python before, so any feed back is more than welcome