Added groovy config slurper for configurable repos

You can now specify a list of bintray repositories to read from. In
~/.lazybones/config.groovy add a line like this:

bintrayRepositories = [
   "kyleboon/lazybones",
   "pledbrook/lazybones-templates"
]

which will allow you to use templates from either repository.
Repositories listed first are higher priority. if 'some-template' exists
in both repositories, it will use the one in the first repository
listed.

list, info and create were all updated to work with multiple
repositories.

If no configuration file is present then the application will use a
default configuration that points to 'pledbrook/lazybones-templates'.
Use 'if' in place of 'try' for flow control.

The app now checks whether the config file exists before trying to load it.
Previously, it attempted the load and then caught the `FileNotFoundException`,
which isn't ideal for an expected condition, i.e. the config file not existing.

The static final fields in `LazybonesMain` have also been capitalised so that
they are more recognisably constants.
Update tests for new error messages in  command.
Change output of `list` command.

Rather than include the host repository with every template name, the `list`
command now  prints the available templates in blocks, one block for each
repository defined in the configuration.
Fix parsing of target directory from arguments.

A bug was introduced in which if the user didn't specify all 3 arguments, the
`create` command would throw an exception. This was because the target dir was
always being initialised from the 3rd argument (even if there were only 2).