Added basic Subsonic interface

app/src/main/java/com/cappielloantonio/play/subsonic/base/SubsonicIncompatibilityException.java

@@ -0,0 +1,13 @@
+package com.cappielloantonio.play.subsonic.base;
+
+public class SubsonicIncompatibilityException extends RuntimeException{
+    private final Version serverApiVersion;
+    private final Version minClientApiVersion;
+
+    public SubsonicIncompatibilityException(Version serverApiVersion, Version minClientApiVersion) {
+        super(String.format("Server API version %s is lower than minimal supported API version %s.", serverApiVersion, minClientApiVersion));
+
+        this.serverApiVersion = serverApiVersion;
+        this.minClientApiVersion = minClientApiVersion;
+    }
+}

app/src/main/java/com/cappielloantonio/play/subsonic/base/Version.java

@@ -0,0 +1,56 @@
+package com.cappielloantonio.play.subsonic.base;
+
+public class Version implements Comparable<Version> {
+
+    private static final String VERSION_PATTERN = "[0-9]+(\\.[0-9]+)*";
+    private final String versionString;
+
+    public static Version of(String versionString) {
+        return new Version(versionString);
+    }
+
+    private Version(String versionString) {
+        if (versionString == null || !versionString.matches(VERSION_PATTERN)) {
+            throw new IllegalArgumentException("Invalid version format");
+        }
+        this.versionString = versionString;
+    }
+
+    public String getVersionString() {
+        return versionString;
+    }
+
+    public boolean isLowerThan(Version version) {
+        return compareTo(version) < 0;
+    }
+
+    @Override
+    public int compareTo(Version that) {
+        if (that == null) {
+            return 1;
+        }
+
+        String[] thisParts = this.getVersionString().split("\\.");
+        String[] thatParts = that.getVersionString().split("\\.");
+
+        int length = Math.max(thisParts.length, thatParts.length);
+
+        for (int i = 0; i < length; i++) {
+            int thisPart = i < thisParts.length ? Integer.parseInt(thisParts[i]) : 0;
+            int thatPart = i < thatParts.length ? Integer.parseInt(thatParts[i]) : 0;
+
+            if (thisPart < thatPart) {
+                return -1;
+            }
+            if (thisPart > thatPart) {
+                return 1;
+            }
+        }
+        return 0;
+    }
+
+    @Override
+    public String toString() {
+        return versionString;
+    }
+}