Upgrade to BitcoinJ 0.12-SNAPSHOT

 - Update imports to reflect BitcoinJ's repackaging, including:

   git grep -l 'import com.google.bitcoin' | \
   xargs perl -p -i -e 's/import com.google.bitcoin/import org.bitcoinj/'

   git grep -l 'com.google.bitcoin' | \
   xargs perl -p -i -e 's/com.google.bitcoin/org.bitcoinj/'

 - Replace use of BitcoinJ's CoinFormat, which has now been renamed to
   MonetaryFormat, using:

   git grep -l 'CoinFormat' | \
   xargs perl -p -i -e 's/CoinFormat/MonetaryFormat/g'

 - Fix calls to BitcoinJ's Script#correctlySpends, whose signature has
   changed from 0.11 => 0.12.

See #98
Upgrade to BitcoinJ 0.12 GA

Resolves #98
Refine README

See #95
Document building from source

Resolves #105
Generate native installers with Gradle JavaFX plugin

The plugin's jfx* tasks tie into the normal Gradle build lifecycle, such
that `gradle build` will now generate executables and installers
according to the OS on which the build is being run. These files are
output to the `build/distributions` directory.

Installers work as expected OS X and Linux at this point.

Windows installers do build, but a very particular configuration is
necessary on the Windows machine doing the building (this configuration
is to be documented in #109). However, even when the configuration is in
place and the MSI installer is successfully built, there is still a
fatal error at installer execution time relating to a missing
msvp100.dll file. See details at
https://bitbucket.org/shemnon/javafx-gradle/issue/43. An issue has been
created to track this from the Bitsquare side as well--see #108.

The changes made in this commit are based on on the samples at
http://bitbucket.org/shemnon/javafx-gradle and the article at
http://jaxenter.com/tutorial-a-guide-to-the-gradle-javafx-plugin-46270.html

The gradle/javafx.gradle file is copied directly from the sources in the
bitbucket repository above, as is apparently the convention (not sure
why this isn't part of the plugin itself, but that's a question to be
addressed later).

Resolves #66, #100
See #108, #109
Update JavaFX Gradle plugin repository config

This commit makes the following improvements upon the stock
javafx.gradle file introduced in the previous commit:

 - Swap Maven Central for JCenter
 - Remove mavenLocal entirely
 - Update naming for clarity

See #66
Add icons based on Apple design guidelines

These icons have been added to the src/deploy/package directory, which
is used by the JavaFX Gradle plugin when building native installers.
Icons are designed primarily for clean integration on OS X systems.
Linux and Windows variants can be added at a later time.

The src/deploy/package/shortcut.sketch directory contains sources used
to design the icons.

See #66, #67
Closes #103
Work around outdated OracleJDK8 on Travis CI server

The Gradle JavaFX plugin requires Oracle JDK 8u20 or better. Travis CI
currently runs something older, and does not have a roadmap for
upgrading. This commit instructs the Travis CI VM to install JDK 8u20
via `apt` prior to actually beginning the build.

See #66
Merge branch 'native'

This merges the work to generate native installers for Windows, Linux
and OS X using the JavaFX Gradle plugin. See #66 for details.