Don't restrict usage to a certain vendor prefix

Make the library work with all browsers that implemented (correctly)
the Web Speech Recognition API, not just the ones that use the webkit
prefix.

This way of using the Speech Recognition API is more practical, espe-
cially as browsers are moving away from vendor prefixes and start using
a flag like mechanism, thus using only the unprefixed version.

http://www.chromium.org/blink/developer-faq#TOC-Will-we-see-a--chrome--vendor-prefix-now-
Merge pull request #38 from alrra/SpeechRecognition

Don't restrict usage to a certain vendor prefix