support floats (in addition to strings) for value of g:apex_API_version
bump default API version up to 28.0
Merge pull request #15 from nwallace/floats_for_api_version

Support both String and Numeric values of g:apex_API_version variable
minor vim doc updates
:ApexLog - shortcut to open log file generated by last Ant command.
added few more annotation to syntax