Added - to keyword characters

This makes syntax highlighting not partially highlight words like default when they appear in identifiers like default-build.
Make strings aware of escaped double quotes

Changed start pattern and added a skip pattern to make jamString region aware of escaped double quote characters.
Fixed error with semicolon errors in strings

A single semicolon in a string literal, such as ";", would be improperly flagged as an error using the old match.  Switching to zero width patterns to match the absence of the proper whitespace before and after the semicolon fixes that behavior.
Merge xiterion's best regex solution from git://github.com/xiterion/jam.vim