Allow the default_errmode to affect datetime parsing
Make the date parser behave liberally in 'none' error mode
Change exception used by json parser error
Update throw tests for changed exception
Optimize the json parser a bit by avoiding dynamic string creation

This change took parsing of a test 44MB json file from 1.2 seconds to
0.8 seconds.
Merge pull request #36 from mwiebe/json_parsetweak

Optimize the json parser a bit by avoiding dynamic string creation
Change the datashape parser to use the shared parse_util