Add Typesafe Releases repository to resolvers
Add sbt-antlr to plugin dependencies
Add icu4j to the library dependencies
Use sbt-antlr plugin
Add volpts.compiler.antlr.generateVolptsDef
Add VolptsDef.g
Add an initial version of VolptsLexer.g
Add library dependencies to corelib and move icu4j from compiler to corelib
Modify the package of VolptsLexer from volpts.compiler.lexer to volpts.compiler.antlr
Fix a bug on SINGLE_LINE_COMMENT
Add 'as' keyword
Add an initial version of VolptsParser.g and ParserUtils
Add an initial version of test.volpts and AntlrSpec
Add keywords 'of' and 'variant'
Fix token ID
Modify compilation_unit to be a list of declarations
Fix 'type' syntax rule
Add declaration syntax rules
Remove data_expr and val_expr and add import_expr and type_expr
Modify the arguments of lambda_expr to be ID, not pat