Sketch Android support (SCL-6273)

Hi,

I decided to give a little nudge with regards to my [long-standing issue](http://youtrack.jetbrains.com/issue/SCL-6273) (originally submitted as https://github.com/JetBrains/intellij-scala/issues/46).

This is still work in progress, to be precise:
* Only [*android-sdk-plugin*](https://github.com/pfn/android-sdk-plugin) is supported (see the discussion in *YouTrack* for motivation).
* The following is added to the `<java>` node (and forces it to be included):

  ```xml
  <orderEntry type="jdk" jdkName="Android API 17 Platform" jdkType="Android SDK"/>
  ```
* A new `<android>` node is added if *android-sdk-plugin* is detected. This is just what should go inside the respective facet. I adapted the [code from *sbt-idea*](https://github.com/mpeltonen/sbt-idea/blob/sbt-0.13/src/main/scala/org/sbtidea/android/AndroidSupport.scala).

  ```xml
  <android>
    <option name="GEN_FOLDER_RELATIVE_PATH_APT" value="./target/android-gen"/>
    <option name="GEN_FOLDER_RELATIVE_PATH_AIDL" value="./target/android-gen"/>
    <option name="MANIFEST_FILE_RELATIVE_PATH" value="./src/main/AndroidManifest.xml"/>
    <option name="RES_FOLDER_RELATIVE_PATH" value="./src/main/res"/>
    <option name="ASSETS_FOLDER_RELATIVE_PATH" value="./src/main/assets"/>
    <option name="LIBS_FOLDER_RELATIVE_PATH" value="./src/main/libs"/>
    <option name="USE_CUSTOM_APK_RESOURCE_FOLDER" value="false"/>
    <option name="CUSTOM_APK_RESOURCE_FOLDER" value=""/>
    <option name="USE_CUSTOM_COMPILER_MANIFEST" value="false"/>
    <option name="CUSTOM_COMPILER_MANIFEST" value=""/>
    <option name="APK_PATH" value="./target/android-bin/.build_integration/name-BUILD-INTEGRATION.apk"/>
    <option name="LIBRARY_PROJECT" value="false"/>
    <option name="RUN_PROCESS_RESOURCES_MAVEN_TASK" value="false"/>
    <option name="GENERATE_UNSIGNED_APK" value="false"/>
    <option name="CUSTOM_DEBUG_KEYSTORE_PATH" value=""/>
    <option name="PACK_TEST_CODE" value="false"/>
    <option name="RUN_PROGUARD" value="false"/>
    <option name="PROGUARD_CFG_PATH" value="/proguard-project.cfg"/>
    <resOverlayFolders>
      <path></path>
    </resOverlayFolders>
    <includeSystemProguardFile>false</includeSystemProguardFile>
    <includeAssetsFromLibraries>false</includeAssetsFromLibraries>
    <additionalNativeLibs/>
  </android>
  ```
Obviously this is not yet picked up by *intellij-scala* or properly tested (but there is a test case).
* Speaking of tests, the current scheme seems to be very unreliable, as the content of `structure / project / build / classes` may vary depending on the global *Sbt* configuration. Not just the entries, but also their order. For example, on my machine `<import>import sbt._, Keys._</import>` goes first, not last.

I would imagine the the separation of *sbt-structure* and *intellij-scala* gives some flexibility in the exchange format. So for example the JDK setting could be included in the `<android>` node, and some extra stuff could be removed.

There are also some tiny changes (`.gitignore`, `README.md`).

Please let me know what you think — this is supposed to be the hard part, as opposed to updating *intellij-scala*.