WIP: $xhr docs, pendingCount()

Features:
- aborting requests
- more flexible callbacks (per status code)
- custom request headers (per request)
- access to response headers
- custom transform functions (both request, response)
- caching
- shortcut methods (get, head, post, put, delete, patch, jsonp)
- exposing pendingCount()

Breaks Takes one parameter now - configuration object
Breaks $xhr.cache removed - use configuration cache: true instead
Breaks $xhr.error, $xhr.bulk removed
Breaks Callback functions accepts parameters: response, status, headers

Closes angular#38
Closes angular#80
Closes angular#180
Closes angular#299
Closes angular#342
Closes angular#395
Closes angular#413
Closes angular#414
Closes angular#507

Author: vojtajina