Document types

Can now specify a document type (e.g., liber, salzinnes, stgallen) and
Neon will parse the mei structure of the document and draw the
appropriate glyphs on the page. Parsing is done for liber and salzinnes
but not for stgallen.

Fixes #74
Update build script

Build script no longer excludes certain drawing library code because it
can now manage multiple document types. For example, if the user wants
to view a liber usualis manuscript but render the neumes like how they
would look in the salzinnes antiphonal, it can be done without
recompiling the javascript code.