Fixed: Improvements to the emscripten build script

Previously the build script would fail with a python error if the emscripten compiler was not found. This commit adds support for checking if the compiler exists on the path, and failing with an error message if it does not.

Also fixed the shebang to use /usr/bin/env bash in case bash was not at /bin/bash.
Fixed: spelling mistake
Updating Readme
Updated README.md to specify verovio.org is ready

This was slightly outdated now that the site is up. :)
Merge pull request #26 from adrianholovaty/patch-1

Thank you!
Merge branch 'master' of https://github.com/rism-ch/verovio

Conflicts:
	README.md
fixing MEI page-based export (was broken)
adding libmei files
Add newly generated libmei that makes verovio compile again
Adjusting emscripten Makefiles
removing old Common att class
updating libmei
Merge pull request #27 from rism-ch/experimental-libmei

Merging experimental libmei to master
Update README.md
Update README.md
Ignoring and logging PPQ duration values
Adding MeterSig class based on LibMEI att class
Adding meterSig element and in score/staffDef
Renaming KeySignature to KeySig
Read intermediate scoreDefs (basic and only for score-based MEI)