Get character using charAt for IE7 compatibility

Using the additional 2 libraries allows for the latest version of proj4js to work in IE7: 

+ https://github.com/es-shims/es5-shim (for the Array.map functions) 
+ http://bestiejs.github.io/json3/  (for the missing JSON function)

However there remains one error in IE7 caused by using the bracket notation to access a character rather than the charAt function which is supported by all browsers. This single change allows proj4js to run in IE7 (as far as I have tested it). 

See http://stackoverflow.com/questions/5943726/string-charatx-or-stringx for more details.