3.0.0: Support for HHVM and PHP7

Added support for PHP7

• Replaced String classes used in test suite as "string" is reserved as
  type name in PHP7.
  (@thekid)
• Rewrote tests verifying exceptions are raised for argument type
  mismatches to work in both PHP 5.x and PHP 7.X.
  (@thekid)

Fixed issue #39: HHVM support

• Fixed sscanf() incompatibilities - HHVM doesn't support %* for not
  assigning variables. Worked around by supplying dummy variables.
  (@thekid)
• Added a native importer which doesn't verify extensions. HHVM does
  not group functions into extensions like PHP does.
  (@thekid)

Removed support for PHP 5.2 and PHP 5.3

• Added new PHP 5.4 the default emitter, which emits new T().method()
  as (new T())->method() instead of wrapping it in the deprecated
  create core functionality.
  (@thekid)
• Removed PHP 5.2 and 5.3 emitters, using -E php5.2 or -E php5.3
  on the command line will produce an error!
  (@thekid)

Other improvements

• Improved performance when loading supported syntaxes by deferring
  parser and lexer instantiation until when first needed.
  (@thekid)