Overhaul property management and main class infrastructure

Use of the Spring Environment
-----------------------------

This change replaces the use of the argparse4j library and basic
Properties objects with the Spring Framework's Environment abstraction.
The Environment allows for managing any number of 'property sources' in
a hierarchical fashion, such that a call to
`environment.getProperty("someKey")` iterates through an ordered set of
property sources, returning the first value associated with the given
key.

BitsquareEnvironment, introduced in this commit, eliminates the
functionality previously present in ConfigLoader, modeling the
bitsquare.conf and bitsquare.properties files as Spring Resource
objects, and in turn creating ResourcePropertySources out of them. These
custom property sources are combined with standard property sources
based on system environment variables and Java system properties as well
as a property source based on the command-line arguments passed to a
Bitsquare application to form a unified, one-stop configuration
hierarchy.

For example, let's say a Bitsquare user wishes to customize the port
that his Bitsquare application listens on. The simplest approach
(assuming the user is comfortable with the command line), would be the
following:

    java -jar bitsquare.jar --port=1234

where '1234' is the custom port of choice. This is convenient enough for
one-off experimentation, but if the user wishes to make this a permanent
arrangement, he may want to add a `port=1234` entry to his
{bitsquare_app_dir}/bitsquare.conf file.

Alternatively, the user may wish to specify the port value as an
environment variable, e.g.:

    PORT=1234 java -jar bitsquare.jar

or with a JVM system property, e.g.:

    java -jar -DPORT=1234 bitsquare.jar

With BitsquareEnvironment, and its customized set of PropertySources in
place, the value of the port property may be specified in any of the
ways described above, and it is all handled in a unified way.

Restructuring of *Main classes
------------------------------

This commit also introduces significant changes to the structure of
executable Bitsquare applications. For example, prior to this change,
the io.bitsquare.app.gui.Main class was responsible for being both a
JavaFX Application and a standard Java main class.

Now, however, these concerns have been renamed and separated.
BitsquareApp is the JavaFX Application, and BitsquareAppMain is the Java
main class. Likewise, BootstrapNode has been broken out into
BootstrapNode and BootstrapNodeMain.

A common base class for the *Main classes has been extracted, named
BitsquareExecutable, which creates a template for option parsing,
environment creation, and ultimately application execution that applies
both to the BootstrapNode and BitsquareApp cases.

Improved help text
------------------

With the removal of argparse4j and the introduction of JOpt for argument
parsing, the application's help text has been improved. Use --help to
display this text, where you'll see information about default values,
etc. To do this easily from the Gradle build, run any of the following
commands:

    # Display help text
    ./gradlew run -Pargs="--help"

    # Qualify the application name as "Bitsquare-Alice"
    ./gradlew run -Pargs="--appName=Alice"

    # Customize the port
    ./gradlew run -Pargs="--port=7377"

Renaming of FatalException
--------------------------

Finally, the exception formerly known as io.bitsquare.gui.FatalException
has been moved up a package and generalized to
io.bitsquare.BitsquareException, as it is now used more widely.