Add an introduction to Crosswalk test suite code structure
Update test case priority in developer guide

- P0 is used for usecase tests for feature to be tested
  and API use cases
- Current P0 tests will be changed to P1
[webapi] Add 22 new TCs for webapi-dlna-xwalk-tests
[webapi] Update netinfo manual case name
[webapi] Add missed image back to canvas
[misc-sanity] Rename sanityapp to usecase
Merge pull request #225 from zqzhang/zqzhang/test-case-priority

Update test case priority in developer guide
Merge pull request #224 from zqzhang/zqzhang/readme

Add an introduction to Crosswalk test suite code structure
Merge pull request #231 from Honry/Honry/canvas-miss-image

[webapi] Add missed image back to canvas
Merge pull request #230 from xinliux/xinliux/webapi/netinfo-manual

[webapi] Update netinfo manual case name
Merge pull request #215 from wanghongjuan/wanghongjuan/webapi/dlna

[webapi] Add 22 new TCs for webapi-dlna-xwalk-tests
[webapi] Add back a test for FileAPI and remove duplicate ones

- new-Blob.html duplicates to Blob-constructor.html
- URL and BlobBuilder interfaces are not in FileAPI spec

Impacted tests: New 1, Update 0, Delete 3
Unit test platform: IVI
Unit test result summary: Pass 1, Fail 0, Blocked 0
Merge pull request #233 from Honry/Honry/rename-sanity

[misc-sanity] Rename sanityapp to usecase
[webdriver] Integrate webdriver tests from W3C web-platform-tests

- These tests have been verified on Ubuntu
- But they are not verified on Crosswalk platforms
- https://crosswalk-project.org/jira/browse/XWALK-1761 is reported
  to track an issue of XwalkDriver on Crosswalk on Tizen IVI
- The README.md shall be improved with instructions of
  how to run the webdriver tests on Crosswalk platforms