Add Gradle build file, setup source directories

* Remove comment and TODO regarding pep257 compliance

* Switch Travis builds to Java

This reverts commit ae63d98.

* Attempt to fix Travis build

* Remove outdated pip modules, update to latest versions

#40: Port script

* Remove pylintrc

* Create SwiftBaseListener
* Ensure command-line arguments are accepted: `gradle run -Pargs="args"`

* TODO: Run `script/antlr` to generate lexer, parser, and listener

#40: Port main

* Extract more string constants to Messages
* Ensure constant names are consistent with text

* Unit tests for SourceFileUtil class

#40: Construct length rule

#40: Line/name length

* Implement AutoCloseable in Printer, to use try-with-resources
* Buffer and dedupe printer messages using a Set, flush on close()
* Resolve TODOs referring to #33 (assertEquals in functional tests)

* Maintains invariant of flushing buffer even if close() is called again

#33: Deduplicate printer messages

* Extract strings to constants

#40: Constant naming rule