Changing dataUri to support map of arraybuffer and json.
Merge branch 'datauri-map' of https://github.com/yosiat/peaks.js into yosiat-datauri-map
Dedicated tests for the XHR loader.
Extracted the XHR requests in two additional waveform.core methods.

It helps readability and testability:
- `getRemoteData`
- `handleRemoteData`

Added a default dataUri format for the sake of backward compatibility.
Less else/if statements as we pick up the URI based on their availability and compatibility.
Testing on content-type, as `responseType` is only valid for compatible XHR2 browsers (aka not Safari 6 and 7 for instance).

- throwing an error if unable to determine a compatible dataUri URI