Fix java.lang.NoSuchMethodError

https://developer.android.com/reference/java/lang/String.html#isEmpty() is available only for Api 11 and not Api 9. 

Better use https://developer.android.com/reference/android/text/TextUtils.html#isEmpty(java.lang.CharSequence)
Merge pull request #19 from bunjix/patch-1

Fix java.lang.NoSuchMethodError
Ignored IDEA metainformation files and update dialog has been made optional.
Modified onUpdateListener in order to be able to reach updates information outside HockeySDK.
Fixed javadoc for missing parameters.
Extracted intent extra strings to avoid magic values and fixed wrong javadoc.
Merge pull request #20 from tuenti/optionalDialog

Optional dialog when updates are discovered.