refactor takeScreenshot method to accept host.
refactor case 'from' and 'to' object.
close all browsers after all cases are executed.
re-design the case structure
refactor case images generator
add new way to close drivers
create Case class
extract getPathKey and getCaseKey into Testcase.
add `beforeEach` handler
refactor takeScreenshots to run method.

in order to integrate browserstack in the future,
need to re-design the structure of viff. Trying to
make viff only consider finding differences from 
image source (browserstack is one of sources).
upgrade to viff 0.7.0 

refactor test cases and compare differences cross 
browsers !!! Merry Chrismas !!!
reconstruct testcase structure.
rename browser argument to capability
use capability to instead only browser name
use capability object to present browser
integrated browserstack
Merge pull request #19 from winsonwq/browserstack_integration

Browserstack integration