bump version number, update change log
support for using env vars to override some desired capabilities values.

switch from firefox to phantom JS for WebdriverFactory local browser test, so it may run on TravisCI

updated change log.
changed the '_DESIRED_CAPABILITIES_ENV_PREFIX' to a public constant so it's more visible and accessible from unit tests.

Added a unit test for the use case of specifying an Appium 'app' desired capabilities.
Update CHANGES.md

formatted changelog entry
Update CHANGES.md
Merge branch 'release/0.4.7' into develop