Support generation of native installer binaries

Use `./gradlew run` to build and run Bitsquare directly.

Use `./gradlew shadowJar` to generate a portable, executable jar.

Use `./gradlew packageNative` to generate an installer binary specific
to the platform you are running on.

In the case of Windows, it is necessary to copy the two .dll files in
the `package/` directory to the directory where Bitsquare.exe is
ultimately installed.

Resolves #243

package/linux.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+set -e
+
+version=$1
+jar=$2
+mainClass=$3
+
+javapackager -deploy \
+    -BappVersion=$version \
+    -Bcategory=Finance \
+    -BlicenseType=GPLv3 \
+    -Bemail=info@bitsquare.io \
+    -native deb \
+    -name Bitsquare \
+    -title Bitsquare \
+    -vendor Bitsquare \
+    -outdir build \
+    -appclass $mainClass \
+    -srcfiles $jar \
+    -outfile Bitsquare
+
+# -Bicon=client/icons/icon.png \