Changed sign in method

Changed this sign in method because there are 2 locators for the same button now (for Mobile and for Desktop environment)
The problem is that the mobile builds fail because the method clicks on the first locator it finds (that will be the desktop one).
We have to find a way to make the sign in method be compatible with Desktop and Mobile environments at the same time.
Here is a screencast:
http://screencast.com/t/ZU5PlDUbbq
Tested this and tests pass on Desktop and mobile