2.1.0: Compile to directory

• Implemented support for mixed type, see issue #28 - @thekid
• Fixed issue #32: Test suite failure on HHVM 3.25 - @thekid
• Allowed trailing commas in grouped use lists as implemented in
  https://wiki.php.net/rfc/list-syntax-trailing-commas
  (@thekid)
• Fixed nullable value types being emitted incorrectly - @thekid
• Merged PR #30: Implement compiling to directory. The command line
  xp compile src/main/php dist/ will compile all source files inside
  the src/main/php directory to dist.
  (@thekid)