Expose Converter class as primary export

[neocotic]

Currently, importing this package provides access to the following methods:

• convert
• convertFile (as of Add CLI & convertFile method to API #2)
• createConverter

The version property is also exposed.

I would like to simplify this further by simply directly exporting the Converter class and move the above methods to static methods on the Converter class. The createConverter method would become redundant, however, it will continue to be used by the static methods as it can be stubbed for unit testing purposes and I'm not sure how easy it is to stub constructors themselves.

The version property is also exposed, which I'd like to make a static constant on the Converter class. Since 0.2.0 will already include a breaking change (#4), I'll take this opportunity to rename this property to VERSION.

This should hopefully simplify the API even further.

[neocotic]

Since the createConverter will be primarily intended for internal use only after these changes, I'll remove it from the README and replace it will details on using a Converter instance instead.

[neocotic]

FYI This enhancement will be reverted in 0.3.0 to be compatible with #17.