use `empty` to check $options['port']

i want use the `port: %%PHINX_DBPORT%%`, but other people maybe like the default port,
so just given a empty port config.
Adds tests for JSON format config files.
Adds support for JSON config files.
Corrects exception message typo.
Index class now supports named indexes.
Added support for named indexes to the MysqlAdapter.
Added support to MysqlAdapter for removing indexes by name.
Added support to SQLiteAdapter for naming indexes and removing indexes by name.
Added support to PostgresAdapter for naming indexes and removing indexes by name.
Added unit tests to the Postgres and SQLite adapters for creating tables with named indexes.
Add tests for DropIndex() with name specified.
Fix PostgresAdapter so DropIndex() works if a name was specified when the index was created.
Merge pull request #196 from archer308/namedindexes

Add support for naming indexes
Merge pull request #194 from cyrusboadway/feature/json-config

JSON Config Support
Merge pull request #188 from xudejian/patch-1

use `empty` to check $options['port']
updating to 0.3.3