Update gradle build files to 0.7.3 android plugin, add Gradle 1.9 wrapper, add information about build to readme

OpenPGP-Keychain/build.gradle

@@ -4,8 +4,7 @@ buildscript {
     }
 
     dependencies {
-        // NOTE: Avoid using dynamic versions (+). This breaks offline builds!
-        classpath 'com.android.tools.build:gradle:0.6.3'
+        classpath 'com.android.tools.build:gradle:0.7.3'
     }
 }
 
@@ -15,9 +14,6 @@ repositories {
     mavenCentral()
 }
 
-/*
- * NOTE: Avoid dependencies from Maven. Include as much sources as possible for F-Droid!
- */
 dependencies {
     compile fileTree(dir: 'libs', includes: ['*.jar'], excludes: ['android-support-v4.jar'])
     compile 'com.android.support:support-v4:19.0.+' // already in actionbarsherlock
@@ -79,4 +75,9 @@ android {
     } else {
         buildTypes.release.signingConfig = null
     }
+    
+    // Do not abort build if lint finds errors
+    lintOptions {
+        abortOnError false
+    }
 }

OpenPGP-Keychain/res/values/strings.xml

@@ -333,7 +333,7 @@
     <string name="import_qr_code_wrong">QR Code malformed! Please try again!</string>
     <string name="import_qr_code_finished">QR Code scanning finished!</string>
     <string name="import_qr_scan_button">Scan QR Code with \'Barcode Scanner\'</string>
-    <string name="import_nfc_text">To exchange keys via NFC, the device needs to be unlocked.</string>
+    <string name="import_nfc_text">To receive keys via NFC, the device needs to be unlocked.</string>
     <string name="import_nfc_help_button">Help</string>
     <string name="import_clipboard_button">Get key from clipboard</string>