unify use of password and secureTextEntry for TextInput

Summary:
Currently, the documentation shows both `password` and `secureTextEntry` as props for TextInput. However, the `password` prop is only passed to the Android component, so it does not work as expected for iOS developers. This PR prefers `password` over `secureTextEntry` but won't break anybody's code.
Closes https://github.com/facebook/react-native/pull/622
Github Author: Will Piers <wpiers@rallydev.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Examples/UIExplorer/TextInputExample.js

@@ -251,7 +251,7 @@ exports.examples = [
       return (
         <View>
           <WithLabel label="true">
-            <TextInput secureTextEntry={true} style={styles.default} value="abc" />
+            <TextInput password={true} style={styles.default} value="abc" />
           </WithLabel>
         </View>
       );