Added and improved classes

Added:

1) InvalidImageException
- Gets thrown when .img could not be identified.

Improved:

1) Command class
- added executeProcessReturnError(String processName, String arg) method

2) Fastboot class
- added JavaDoc comments
- added oemUnlockBootloader() method
- added oemLockBootloader() method
- added rebootToAndroid() method
- added rebootBootloader() method
- added flashImg(FastbootImgType imgType, String imgLocation) method