switch to gradle

Hi,

with this pull request you switch to gradle and build all required libs from source. 
I have forked the bouncycastle and otr4j and make it build with your projekt. 
You should  possibly fork it to your repository and adjust the submodule.

You can use it with [Android Studio](https://developer.android.com/sdk/installing/studio.html)

* add submodule for [otr4j](https://github.com/betheg/otr4j/tree/conversations)
* add submodule for [bouncycastle](https://github.com/betheg/bc-java/tree/conversations) tag r1rv51
* update [minidns](https://github.com/rtreffer/minidns/tree/0.1.3) to tag 0.1.3

To sign release build, create file `gradle.properties` in 
USER_HOME/.gradle or in your project root directory with this content:
```
mStoreFile=/path/to/key.store
mStorePassword=xxx
mKeyAlias=alias
mKeyPassword=xxx
```
to test it just do
`./gradlew assembleDebug`

the apk is in
`build/outputs/apk/Conversations-debug-0.8.apk`