add characterisation tests for Parser
fix getFixBytes boundary checking

in case there we ask for more bytes than there are in buffer, get()
will raise SyntaxError exception while getFixBytes won't

this causes the code depending on getFixBytes doing boundary checks
(and by extension getVarBytes) to be unreliable
Merge pull request #31 from tomato42/parser-fixes

Parser fix for getFixBytes (Hubert Kario)